Some Useful Links: Quantum Gravity Seminar by John Baez

I have not been able to make posts tackling physics in a while, since I have lately been focusing my efforts on some purely mathematical stuff which I’m trying very hard to understand. Hence my last few posts have been quite focused mostly on algebraic geometry and category theory. Such might perhaps be the trend in the coming days, although of course I still want to make more posts on physics at some point.

Of course, the “purely mathematical” stuff I’ve been posting about is still very much related to physics. For instance, in this post I’m going to link to a webpage collecting notes from seminars by mathematical physicist John Baez on the subject of quantum gravity – and much of it involves concepts from subjects like category theory and algebraic topology (for more on the basics of these subjects from this blog, see Category TheoryHomotopy Theory, and Homology and Cohomology).

Here’s the link:

Seminar by John Baez

As Baez himself says on the page, however, quantum gravity is not the only subject tackled on his seminars. Other subjects include topological quantum field theory, quantization, and gauge theory, among many others.

John Baez also has lots of other useful stuff on his website. One of the earliest mathematics and mathematical physics blogs on the internet is This Week’s Finds in Mathematical Physics, which apparently goes back all the way to 1995, and is one of the inspirations for this blog:

This Week’s Finds in Mathematical Physics by John Baez

Many of the posts on This Week’s Finds in Mathematical Physics show the countless fruitful, productive, and beautiful interactions between mathematics and physics. This is also one of the main goals of this blog – reflected even by the posts which have been focused on mostly “purely mathematical” stuff.

Monoidal Categories and Monoids

A monoid is a concept in mathematics similar to that of a group (see Groups), except that every element need not have an inverse. Therefore, a monoid is a set, equipped with a law of composition which is associative, and an identity element. An example of a monoid is the natural numbers (including zero) with the law of composition given by addition.

In this post, we will introduce certain concepts in category theory (see Category Theory) that are abstractions of the classical idea of a monoid.

A monoidal category is given by a category \mathbf{C}, a bifunctor \Box: \mathbf{C}\times\mathbf{C}\rightarrow\mathbf{C}, an object I of \mathbf{C}, and three natural isomorphisms \alpha (also known as the associator), \lambda (also known as the left unitor), and \rho (also known as the right unitor), with components

\displaystyle \alpha_{A,B,C}:A\Box (B\Box C)\cong (A\Box B)\Box C

\displaystyle \lambda_{A}:I\Box A\cong A

\displaystyle \rho_{A}:A\Box I\cong A

satisfying the conditions

\displaystyle 1_{A}\Box\alpha_{A,B,C}\circ\alpha_{A,B\Box C,D}\circ\alpha_{A,B,C}\Box 1_{D}=\alpha_{A,B,C\Box D}\circ\alpha_{A\Box B,C,D}

for any four objects A, B, C, and D of \mathbf{C}, and

\displaystyle \alpha_{A,I,B}\circ 1_{A}\Box \lambda_{B}=\rho_{A}\Box 1_{B}

for any two objects A and B in \mathbf{C}.

The following “commutative diagrams” courtesy of user IkamusumeFan of Wikipedia may help express these conditions better (the symbol \otimes is used here instead of \Box to denote the bifunctor; this is very common notation, but we use \Box following the book Categories for the Working Mathematician by Saunders Mac Lane in order to differentiate it from the tensor product, which is just one specific example of the bifunctor in question; I hope this will not cause any confusion):

Monoidal_category_pentagon.svg

Monoidal_category_triangle.svg

If the natural isomorphisms \alpha, \lambda, and \rho are identities, then we have a strict monoidal category.

A monoid object, or monoid in a monoidal category (\mathbf{C},\Box,I) is an object M of \mathbf{C} together with two morphisms \mu:M\Box M\rightarrow M and \eta:I\rightarrow M satisfying the conditions

\displaystyle \mu\circ 1\Box\mu\circ\alpha=\mu\circ\mu\Box 1

\displaystyle \mu\circ \eta\Box 1=\lambda

\displaystyle \mu\circ 1\Box\eta=\rho

Again we can use the following commutative diagrams made by User IkamusumeFan of Wikipedia to help express these conditions:

Monoid_multiplication.svg

Monoid_unit_svg.svg

As examples of monoidal categories, we have the following:

\displaystyle (\mathbf{Set},\times,1)

\displaystyle (\mathbf{Ab},\otimes,\mathbb{Z})

\displaystyle (K\mathbf{-Mod},\otimes_{K},K)

(\mathbf{Cat},\times,\mathbf{1})

(\mathbf{C}^{\mathbf{C}},\circ,\text{Id})    (\mathbf{C}^{\mathbf{C}} denotes the category of functors from \mathbf{C} to itself)

The monoids in these monoidal categories are given respectively by the following:

Ordinary monoids

Rings

K-algebras

Strict monoidal categories

Monads (see Adjoint Functors and Monads)

Among the important kinds of monoidal categories with extra structure are braided monoidal categories and symmetric monoidal categories. A braided monoidal category \mathbf{C} is a monoidal category equipped with a natural isomorphism \gamma (also known as a commutativity constraint) with components \gamma_{A,B}:A\Box B\cong B\Box A satisfying the following coherence conditions

\displaystyle \alpha_{B,C,A}\circ\gamma_{A,B\Box C}\circ\alpha_{A,B,C}=1_{B}\Box\gamma_{A,C}\circ\alpha_{B,A,C}\circ \gamma_{A,B}\Box 1_{C}

\displaystyle \alpha_{C,A,B}^{-1}\circ\gamma_{A\Box B,C}\circ\alpha_{A,B,C}^{-1}=\gamma_{A,C}\Box 1_{B}\circ\alpha_{A,C,B}^{-1}\circ 1\Box\gamma_{A}\gamma_{A,B}

which can be expressed in the following commutative diagrams (once again credit goes to User IkamusumeFan of Wikipedia):

Braid_category_hexagon.svg

Braid_category_inverse_hexagon.svg

The category \mathbf{C} is a symmetric monoidal category if the isomorphisms \gamma_{A,B} satisfy the condition \gamma_{B,A}\circ\gamma_{A,B}=1_{A\Box B}. We have already encountered an example of this category in The Theory of Motives in the form of tensor categories, defined as a symmetric monoidal categories whose Hom-sets (the sets of morphisms from a fixed object A to another object B) form a vector space (the term “tensor category” is sometimes used to refer to other concepts in mathematics though, including symmetric monoidal categories themselves).

Another important kind of monoidal category is a closed monoidal category. A closed monoidal category is a monoidal category where the functor -\Box B has a right adjoint (see Adjoint Functors and Monads) also known as the “internal Hom functor”, which is like a Hom functor that takes values in the category itself instead of in sets, and is denoted by (\ )^{B}. We have already seen an example of a closed monoidal category in Adjoint Functors and Monads, given by the category of R-modules for a fixed commutative ring R. There A^{B} was given by \text{Hom}(A,B) (this is the set of R-linear transformations from A to B, which itself is an R-module).

We see therefore that the concepts of monoidal categories and monoids can be found everywhere in mathematics. Studying these structures are not only interesting for their own sake, but can also help us find or construct other useful new concepts in mathematics.

References:

Monoidal Category on Wikipedia

Monoid on Wikipedia

Braided Monoidal Category on Wikipedia

Symmetric Monoidal Category on Wikipedia

Closed Monoidal Category on Wikipedia

Image by User IkamusumeFan of Wikipedia

Image by User IkamusumeFan of Wikipedia

Image by User IkamusumeFan of Wikipedia

Image by User IkamusumeFan of Wikipedia

Image by User IkamusumeFan of Wikipedia

Image by User IkamusumeFan of Wikipedia

Categories for the Working Mathematician by Saunders Mac Lane

The Yoneda Lemma

Update: Some time after I published this post, I came across the following post on another blog that makes for a really nice intuitive introduction to the ideas expressed by the Yoneda lemma:

The Most Obvious Secret in Mathematics at Math3ma

I must admit that my own post might not offer much in the way of intuition (and really could have been written better), so I highly recommend reading the above link in conjunction with this one.

In Algebraic Spaces and Stacks we introduced the notion of a representable functor, and we made use of it to “transfer” the properties of schemes to functors and categories over some fixed category. In this short post we discuss an important related concept, one of the most important concepts in category theory, called the Yoneda lemma.

Let \mathbf{C} be a category, and let A be any object of \mathbf{C}. The Yoneda lemma states that the set of natural transformations from the functor \text{Hom}(-,A) to any contravariant functor G from \mathbf{C} to the category of sets is in bijection with the set G(A).

In the case that G is the contravariant functor \text{Hom}(-,B), where B is an element of \mathbf{C}, the Yoneda lemma says that the set of natural transformations from \text{Hom}(-,A) to \text{Hom}(-,B) is in bijection with the set \text{Hom}(A,B).

We can treat the functor \text{Hom}(-,-) as a covariant functor from the category \mathbf{C} to the category \textbf{Sets}^{\mathbf{C}^{\text{op}}} of contravariant functors from \mathbf{C} to the category of sets, which sends an object A of \mathbf{C} to the contravariant functor \text{Hom}(-,A), and a morphism f:A\rightarrow B of \mathbf{C} to the natural transformation \text{Hom}(-,f):\text{Hom}(-,A)\rightarrow\text{Hom}(-,B). Then the Yoneda lemma, via the result given in the preceding paragraph, says that the functor \text{Hom}(-,-): \mathbf{C}\rightarrow\mathbf{Sets}^{\mathbf{C}^{\text{op}}} is fully faithful. We also say that this functor is an embedding; in particular, it is called the Yoneda embedding. It embeds the category \mathbf{C} into the category \mathbf{Sets}^{\mathbf{C}^{\text{op}}}.

The Yoneda lemma is an important ingredient of the functor of points approach to the theory of schemes. Furthermore, the Yoneda lemma tells us that the category of schemes is embedded as a subcategory of the category of contravariant functors from the category of schemes to the category of sets, so we can also try looking at a bigger subcategory of the latter category, and see if we can come up with interesting objects to study – this actually leads us to the theory of algebraic spaces.

References:

Yoneda Lemma on Wikipedia

Yoneda Lemma on nLab

Representable Functors on Rigorous Trivialities

The Most Obvious Secret in Mathematics at Math3ma

Localization and Gromov-Witten Invariants by Kai Behrend

Categories for the Working Mathematician by Saunders Mac Lane

Adeles and Ideles

In Valuations and Completions we introduced the p-adic numbers \mathbb{Q}_{p}, which, like the real numbers, are the completion of the rational numbers under a certain kind of valuation. There is one such valuation for each prime number p, and another for the “infinite prime”, which is just the usual absolute value. Each valuation may be thought of as encoding number theoretic information related to the prime p, or to the “infinite prime”, for the case of the absolute value (more technically, the p-adic valuations are referred to as nonarchimedean valuations, while the absolute value is an example of an archimedean valuation).

We can consider valuations not only for the rational numbers, but for more general algebraic number fields as well. In its abstract form, given an algebraic number field K, a (multiplicative) valuation of K is simply any function |\ | from K to \mathbb{R} satisfying the following properties:

(i) |x|\geq 0, where x=0 if and only if x=0

(ii) |xy|=|x||y|

(iii) |x+y|\leq|x|+|y|

If this seems reminiscent of the discussion in Metric, Norm, and Inner Product, it is because a valuation does, in fact, define a metric on K, and by extension, a topology. Two valuations are equivalent if they define the same topology; another way to phrase this statement is that two valuations |\ |_{1} and |\ |_{2} are equivalent if |x|_{1}=|x|_{2}^{s} for some positive real number s, for all x\in K.  The valuation is nonarchimedean if |x+y|\leq\text{max}\{|x|,|y|\}; otherwise, it is archimedean.

Just as in the case of rational numbers, we also have an exponential valuation, defined as a function v from the field K to \mathbb{R}\cup \infty satisfying the following conditions:

(i) v(x)=\infty if and only if x=0

(ii) v(xy)=v(x)+v(y)

(iii) v(x+y)\geq\text{min}\{v(x),v(y)\}

Two exponential valuations v_{1} and v_{2} are equivalent if v_{1}(x)=sv_{2}(x) for some real number s, for all x\in K.

The idea of valuations allows us to make certain concepts in algebraic number theory (see Algebraic Numbers) more abstract. We define a place v of an algebraic number field K as an equivalence class of valuations of K. We write K_{v} to denote the completion of K under the place v; these are the generalizations of the p-adic numbers and real numbers to algebraic number fields other than \mathbb{Q}. The nonarchimedean places are also called the finite places, while the archimedean places are also called the infinite places. To express whether a place v is a finite place or an infinite place, we write v|\infty or v\nmid\infty respectively.

The infinite places are of two kinds; the ones for which K_{v} is isomorphic to \mathbb{R} are called the real places, while the ones for which K_{v} is isomorphic to \mathbb{C} are called the complex places. The number of real places and complex places of K, denoted by r_{1} and r_{2} respectively, satisfy the equation r_{1}+2r_{2}=n, where n is the degree of K over \mathbb{Q}, i.e. n=[K:\mathbb{Q}].

By the way, in some of the literature, such as in the book Algebraic Number Theory by Jurgen Neukirch, “places” are also referred to as “primes“. This is intentional – one may actually think of our definition of places as being like a more abstract replacement of the definition of primes. This is quite advantageous in driving home the concept of primes as equivalence classes of valuations; however, to avoid confusion, we will stick to using the term “places” here, along with its corresponding notation.

When v is a nonarchimedean valuation, we let \mathfrak{o}_{v} denote the set of all elements x of K_{v} for which |x|_{v}\leq 1. It is an example of a ring with special properties called a valuation ring. This means that, for any x in K, either x or x^{-1} must be in \mathfrak{o}_{v}. We let \mathfrak{o}_{v}^{*} denote the set of all elements of \mathfrak{o}_{v} for which |x|_{v}=1, and we let \mathfrak{p}_{v} denote the set of all elements of \mathfrak{o}_{v} for which |x|_{v}< 1. It is the unique maximal ideal of \mathfrak{o}_{v}.

Now we proceed to consider the modern point of view in algebraic number theory, which is to consider all these equivalence classes of valuations together. This will lead us to the language of adeles and ideles.

An adele \alpha of K is a family (\alpha_{v}) of elements \alpha_{v} of K_{v} where \alpha_{v}\in K_{v}, and \alpha_{v}\in\mathfrak{o}_{v} for all but finitely many v. We can define addition and multiplication componentwise on adeles, and the resulting ring of adeles is then denoted \mathbb{A}_{K}. The group of units of the ring of adeles is called the group of ideles, denoted I_{K}. For a finite set of primes S that includes the infinite primes, we let

\displaystyle \mathbb{A}_{K}^{S}=\prod_{v\in S}K_{v}\times\prod_{v\notin S}\mathfrak{o}_{v}

and

\displaystyle I_{K}^{S}=\prod_{v\in S}K_{v}^{*}\times\prod_{v\notin S}\mathfrak{o}_{v}^{*}.

We denote the set of infinite primes by S_{\infty}. Then \mathfrak{o}_{K}, the ring of integers of the number field K, is given by K\cap\mathbb{A}_{K}^{S_{\infty}}, while \mathfrak{o}_{K}^{*}, the group of units of \mathfrak{o}_{K}, is given by K^{*}\cap I_{K}^{S_{\infty}}.

Any element of K is also an element of \mathbb{A}_{K}, and any element of K^{*} (the group of units of K) is also an element of I_{K}. The elements of I_{K} which are also elements of K^{*} are called the principal ideles. This should not be confused with the concept of principal ideals; however the terminology is perhaps suggestive on purpose. In fact, ideles and fractional ideals are related. Any fractional ideal \mathfrak{a} can be expressed in the form

\displaystyle \mathfrak{a}=\prod_{\mathfrak{p}}\mathfrak{p}^{\nu_{\mathfrak{p}}}.

Therefore, we have a mapping

\displaystyle \alpha\mapsto (\alpha)=\prod_{\mathfrak{p}}\mathfrak{p}^{v_{\mathfrak{p}}(\alpha_v)}

from the group of ideles to the group of fractional ideals. This mapping is surjective, and its kernel is I_{K}^{S_{\infty}}.

The quotient group I_{K}/K^{*} is called the idele class group of K, and is denoted by C_{K}. Again, this is not to be confused with the ideal class group we discussed in Algebraic Numbers, although the two are related; in the language of ideles, the ideal class group is defined as I_{K}/I_{K}^{S_{\infty}}K^{*}, and is denoted by Cl_{K}. There is a surjective homomorphism C_{K}\mapsto Cl_{K} induced by the surjective homomorphism from the group of ideles to the group of fractional ideals that we have described in the preceding paragraph.

An important aspect of the concept of adeles and ideles is that they can be equipped with topologies (see Basics of Topology and Continuous Functions). For the adeles, this topology is generated by the neighborhoods of 0 in \mathbb{A}_{K}^{S_{\infty}} under the product topology. For the ideles, this topology is defined by the condition that the mapping \alpha\mapsto (\alpha,\alpha^{-1}) from I_{K} into \mathbb{A}_{K}\times\mathbb{A}_{K} be a homeomorphism onto its image. Both topologies are locally compact, which means that every element has a neighborhood which is compact, i.e. every open cover of that neighborhood has a finite subcover. For the group of ideles, its topology is compatible with its group structure, which makes it into a locally compact topological group.

In this post, we have therefore seen how the theory of valuations can allow us to consider a more abstract viewpoint for algebraic number theory, and how considering all the valuations together to form adeles and ideles allows us to rephrase the usual concepts related to algebraic number fields, such as the ring of integers, its group of units, and the ideal class group, in a new form. In addition, the topologies on the adeles and ideles can be used to obtain new results; for instance, because the group of ideles is a locally compact topological (abelian) group, we can use the methods of harmonic analysis (see Some Basics of Fourier Analysis) to study it. This is the content of the famous thesis of the mathematician John Tate. Another direction where the concept of adeles and ideles can take us is class field theory, which relates the idele class group to the other important group in algebraic number theory, the Galois group (see Galois Groups). The language of adeles and ideles can also be applied not only to algebraic number fields but also to function fields of curves over finite fields. Together these fields are also known as global fields.

References:

Adele Ring on Wikipedia

Tate’s Thesis on Wikipedia

Class Field Theory on Wikipedia

Algebraic Number Theory by Jurgen Neukirch

Algebraic Number Theory by J. W. S. Cassels and A. Frohlich

A Panorama of Pure Mathematics by Jean Dieudonne

Adjoint Functors and Monads

In Category Theory we introduced the language of categories, and in many posts in this blog we have seen how useful it is in describing concepts in modern mathematics, for example in the two most recent posts, The Theory of Motives and Algebraic Spaces and Stacks. In this post, we introduce another important concept in category theory, that of adjoint functors, as well as the closely related notion of monads. Manifestations of these ideas are quite ubiquitous in modern mathematics, and we enumerate a few examples in this post.

An adjunction between two categories \mathbf{C} and \mathbf{D} is a pair of functors, F:\mathbf{C}\rightarrow \mathbf{D}, and G:\mathbf{D}\rightarrow \mathbf{C}, such that there exists a bijection

\displaystyle \text{Hom}_{\mathbf{D}}(F(X),Y)\cong\text{Hom}_{\mathbf{C}}(X,G(Y))

for all objects X of \mathbf{C} and all objects Y of \mathbf{D}. We say that F is left-adjoint to G, and that G is right-adjoint to F. We may also write F\dashv G.

An adjunction determines two natural transformations \eta: 1_{\mathbf{C}}\rightarrow G\circ F and \epsilon:F\circ G\rightarrow 1_{\mathbf{D}}, called the unit and counit, respectively. Conversely, the functors F and G, together with the natural transformations \eta and \epsilon, are enough to determine the adjunction, therefore we can also denote the adjunction by (F,G,\eta,\epsilon).

We give an example of an adjunction. Let K be a fixed field, and consider the functors

F:\textbf{Sets}\rightarrow\textbf{Vect}_{K}

\displaystyle G:\textbf{Vect}_{K}\rightarrow\textbf{Sets}

where F is the functor which assigns to a set X the vector space F(X) made up of formal linear combinations of elements of X with coefficients in K; in other words, an element of F(X) can be written as \sum_{i}a_{i}x_{i}, where a_{i}\in K and x_{i}\in X, and G is the forgetful functor, which assigns to a vector space V the set G(V) of elements (vectors) of V; in other words it simply “forgets” the vector space structure on V.

For every function g:X\rightarrow G(V) in \textbf{Sets} we have a linear transformation f:F(X)\rightarrow V in \textbf{Vect}_{K} given by f(\sum_{i}a_{i}x_{i})=\sum_{i}a_{i}g(x_{i}). The correspondence \psi:g\rightarrow f has an inverse \varphi, given by restricting f to X (so that our only linear transformations are of the form f(x_{i}), and we can obtain set-theoretic functions corresponding to these linear transformations). Hence we have a bijection

\displaystyle \text{Hom}_{\textbf{Vect}_{K}}(F(X),V)\cong\text{Hom}_{\textbf{Sets}}(X,G(V)).

We therefore see that the two functors F and G form an adjunction; the functor F (sometimes called the free functor) is left-adjoint to the forgetful functor G, and G is right-adjoint to F.

As another example, consider now the category of modules over a commutative ring R, and the functors -\otimes_{R}B and \text{Hom}_{R}(B,-) (see The Hom and Tensor Functors). For every morphism g:A\otimes_{R}B\rightarrow C we have another morphism f: A\rightarrow\text{Hom}_{R}(B,C) given by [f(a)](b)=g(a,b). We actually have a bijection

\displaystyle \text{Hom}(A\otimes_{R}B,C)\cong\text{Hom}(A,\text{Hom}_{R}(B,C)).

This is called the Tensor-Hom adjunction.

Closely related to the concept of an adjunction is the concept of a monad. A monad is a triple (T,\eta,\mu) where T is a functor from \mathbf{C} to itself, \eta is a natural transformation from 1_{\mathbf{C}} to T, and \mu is a natural transformation from \mu:T^{2}\rightarrow T, satisfying the following properties:

\displaystyle \mu\circ\mu_{T}=\mu\circ T\mu

\displaystyle \mu\circ\eta_{T}=\mu\circ T\eta=1

Dual to the concept of a monad is the concept of a comonad. A comonad on a category \mathbf{C} may be thought of as a monad on the opposite category \mathbf{C}^{\text{op}}.

As an example of a monad, we can consider the action of a fixed group G on a set (such as the symmetric group permuting the elements of the set, for example). In this case, our category will be \mathbf{Sets}, and T, \eta, and \mu are given by

\displaystyle T(X)=G\times X

\displaystyle \eta:X\rightarrow G\times X given by x\rightarrow\langle g,x\rangle

\displaystyle \mu:G\times (G\times X)\rightarrow G\times X given by \langle g_{1},\langle g_{2},x\rangle\rangle\rightarrow \langle g_{1}g_{2},x\rangle

Adjunctions and monads are related in the following way. Let F:\mathbf{C}\rightarrow\mathbf{D} and G:\mathbf{D}\rightarrow\mathbf{C} be a pair of adjoint functors with unit \eta and counit \epsilon. Then we have a monad on \mathbf{C} given by (G\circ F,\eta,G\epsilon_{F}). We can also obtain a comonad given by (F\circ G,\epsilon,F\eta_{G}).

Conversely, if we have a monad (T,\eta,\mu) on the category \mathbf{C}, we can obtain a pair of adjoint functors F:\mathbf{C}\rightarrow\mathbf{C}^{T} and G:\mathbf{C}^{T}\rightarrow\mathbf{C}, where \mathbf{C}^{T} is the Eilenberg-Moore category, whose objects (called T-algebras) are pairs (A,\alpha), where A is an object of \mathbf{C}, and \alpha is a morphism T(A)\rightarrow A satisfying

\displaystyle \alpha\circ \eta_{A}=1_{A}

\displaystyle \alpha\circ \mu_{A}=\alpha\circ T(\alpha),

and whose morphisms h:(A,\alpha)\rightarrow (B,\beta) are morphisms h:A\rightarrow B in \mathbf{C} such that

\displaystyle h\circ\alpha=\beta\circ T(h).

In the example we gave above in the discussion on monads, the T-algebras are exactly the sets with the action of the group G. If X is such a set, then the corresponding T-algebra is the pair (X,h), where the function h:G\times X\rightarrow X satisfies

\displaystyle h(g_{1},h(g_{2},x))=h(g_{1}g_{2},x)

\displaystyle h(e,x)=x.

For comonads, we have a dual notion of coalgebras. These “dual” ideas are important objects of study in themselves, for example in topos theory. Another reason to consider comonads and coalgebras is that in mathematics there often arises a situation where we have three functors

\displaystyle L:\mathbf{D}\rightarrow\mathbf{C}

\displaystyle F:\mathbf{C}\rightarrow\mathbf{D}

\displaystyle R:\mathbf{D}\rightarrow\mathbf{C}

where L is left-adjoint to F, and R is right-adjoint to F (a so-called adjoint triple). As an example, consider the forgetful functor F:\textbf{Top}\rightarrow\textbf{Sets} which assigns to a topological space its underlying set. It has both a left-adjoint L:\textbf{Sets}\rightarrow\textbf{Top} which assigns to a set X the trivial topology (where the only open sets are the empty set and X itself), and a right-adjoint R:\textbf{Sets}\rightarrow\textbf{Top} which assigns to the set X the discrete topology (where every subset of X is an open set). Therefore we have a monad and a comonad on \textbf{Sets} given by F\circ L and F\circ R respectively.

Many more examples of adjoint functors and monads can be found in pretty much all areas of mathematics. And according to a principle attributed to the mathematician Saunders Mac Lane (one of the founders of category theory, along with Samuel Eilenberg), such a structure that occurs widely enough in mathematics deserves to be studied for its own sake.

References:

Adjoint Functors on Wikipedia

Monad on Wikipedia

Adjoint Functor on nLab

Adjunction on nLab

Monad on nLab

Adjunctions by TheCatsters on YouTube

Monads by TheCatsters on YouTube

Categories for the Working Mathematician by Saunders Mac Lane

Category Theory by Steve Awodey

Algebraic Spaces and Stacks

We introduced the concept of a moduli space in The Moduli Space of Elliptic Curves, and constructed explicitly the moduli space of elliptic curves, using the methods of complex analysis. In this post, we introduce the concepts of algebraic spaces and stacks, far-reaching generalizations of the concepts of varieties and schemes (see Varieties and Schemes Revisited), that are very useful, among other things, for constructing “moduli stacks“, which are an improvement over the naive notion of moduli space, namely in that one can obtain from it all “families of objects” by pulling back a “universal object”.

We need first the concept of a fibered category (also spelled fibred category). Given a category \mathcal{C}, we say that some other category \mathcal{S} is a category over \mathcal{C} if there is a functor p from \mathcal{S} to \mathcal{C} (this should be reminiscent of our discussion in Grothendieck’s Relative Point of View).

If \mathcal{S} is a category over some other category \mathcal{C}, we say that it is a fibered category (over \mathcal{C}) if for every object U=p(x) and morphism f: V\rightarrow U in \mathcal{C}, there is a strongly cartesian morphism \phi: f^{*}x\rightarrow x in \mathcal{S} with f=p(\phi).

This means that any other morphism \psi: z\rightarrow x whose image p(\psi) under the functor p factors as p(\psi)=p(\phi)\circ h must also factor as \psi=\phi\circ \theta under some unique morphism \theta: z\rightarrow f^{*}x whose image under the functor p is h. We refer to f^{*}x as the pullback of x along f.

Under the functor p, the objects of \mathcal{S} which get sent to U in \mathcal{C} and the morphisms of \mathcal{S} which get sent to the identity morphism i_{U} in \mathcal{C} form a subcategory of \mathcal{S} called the fiber over U. We will also write it as \mathcal{S}_{U}.

An important example of a fibered category is given by an ordinary presheaf on a category \mathcal{C}, i.e. a functor F:\mathcal{C}^{\text{op}}\rightarrow (\text{Set}); we can consider it as a category fibered in sets \mathcal{S}_{F}\rightarrow\mathcal{C}.

A special kind of fibered category that we will need later on is a category fibered in groupoids. A groupoid is simply a category where all morphisms have inverses, and a category fibered in groupoids is a fibered category where all the fibers are groupoids. A set is a special kind of groupoid, since it may be thought of as a category whose only morphisms are the identity morphisms (which are trivially their own inverses). Hence, the example given in the previous paragraph, that of a presheaf, is also an example of a category fibered in groupoids, since it is fibered in sets.

Now that we have the concept of fibered categories, we next want to define prestacks and stacks. Central to the definition of prestacks and stacks is the concept known as descent, so we have to discuss it first. The theory of descent can be thought of as a formalization of the idea of “gluing”.

Let \mathcal{U}=\{f_{i}:U_{i}\rightarrow U\} be a covering (see Sheaves and More Category Theory: The Grothendieck Topos) of the object U of \mathcal{C}. An object with descent data is a collection of objects X_{i} in \mathcal{S}_{U} together with transition isomorphisms \varphi_{ij}:\text{pr}_{0}^{*}X_{i}\simeq\text{pr}_{1}^{*}X_{j} in \mathcal{S}_{U_{i}\times_{U}U_{j}}, satisfying the cocycle condition

\displaystyle \text{pr}_{02}^{*}\varphi_{ik}=\text{pr}_{01}^{*}\varphi_{ij}\circ \text{pr}_{12}^{*}\varphi_{jk}:\text{pr}_{0}^{*}X_{i}\rightarrow \text{pr}_{2}^{*}X_{k}

The morphisms \text{pr}_{0}:U_{i}\times_{U}U_{j}\rightarrow U_{i} and the \text{pr}_{1}:U_{i}\times_{U}U_{j}\rightarrow U_{j} are the projection morphisms. The notations \text{pr}_{0}^{*}X_{i} and \text{pr}_{1}^{*}X_{j} means that we are “pulling back” X_{i} and X_{j} from \mathcal{S}_{U_{i}} and \mathcal{S}_{U_{j}}, respectively, to \mathcal{S}_{U_{i}\times_{U}U_{j}}.

A morphism between two objects with descent data is a a collection of morphisms \psi_{i}:X_{i}\rightarrow X'_{i} in \mathcal{S}_{U_{i}} such that \varphi'_{ij}\circ\text{pr}_{0}^{*}\psi_{i}=\text{pr}_{1}^{*}\psi_{j}\circ\varphi_{ij}. Therefore we obtain a category, the category of objects with descent data, denoted \mathcal{DD}(\mathcal{U}).

We can define a functor \mathcal{S}_{U}\rightarrow\mathcal{DD}(\mathcal{U}) by assigning to each object X of \mathcal{S}_{U} the object with descent data given by the pullback f_{i}^{*}X and the canonical isomorphism \text{pr}_{0}^{*}f_{i}^{*}X\rightarrow\text{pr}_{1}^{*}f_{j}^{*}X. An object with descent data that is in the essential image of this functor is called effective.

Before we give the definitions of prestacks and stacks, we recall some definitions from category theory:

A functor F:\mathcal{A}\rightarrow\mathcal{B} is faithful if the induced map \text{Hom}_{\mathcal{A}}(x,y)\rightarrow \text{Hom}_{\mathcal{B}}(F(x),F(y)) is injective for any two objects x and y of \mathcal{A}.

A functor F:\mathcal{A}\rightarrow\mathcal{B} is full if the induced map \text{Hom}_{\mathcal{A}}(x,y)\rightarrow \text{Hom}_{\mathcal{B}}(F(x),F(y)) is surjective for any two objects x and y of \mathcal{A}.

A functor F:\mathcal{A}\rightarrow\mathcal{B} is essentially surjective if any object y of \mathcal{B} is isomorphic to the image F(x) of some object x in \mathcal{A} under F.

A functor which is both faithful and full is called fully faithful. If, in addition, it is also essentially surjective, then it is called an equivalence of categories.

Now we give the definitions of prestacks and stacks using the functor \mathcal{S}_{U}\rightarrow\mathcal{DD}(\mathcal{U}) we have defined earlier.

If the functor \mathcal{S}_{U}\rightarrow\mathcal{DD}(\mathcal{U}) is fully faithful, then the fibered category \mathcal{S}\rightarrow\mathcal{C} is a prestack.

If the functor \mathcal{S}_{U}\rightarrow\mathcal{DD}(\mathcal{U}) is an equivalence of categories, then the fibered category \mathcal{S}\rightarrow\mathcal{C} is a stack.

Going back to the example of a presheaf as a fibered category, we now look at what it means when it satisfies the conditions for being a prestack, or a stack:

(i) F is a prestack if and only if it is a separated functor,

(ii) F is stack if and only if it is a sheaf.

We now have the abstract idea of a stack in terms of category theory. Next we want to have more specific examples of interest in algebraic geometry, namely, algebraic spaces and algebraic stacks. For this we need first the idea of a representable functor (and the closely related idea of a representable presheaf). The importance of representability is that this will allow us to “transfer” interesting properties of morphisms between schemes such as being surjective, etale, or smooth, to functors between categories or natural transformations between functors. Therefore we will be able to say that a functor or natural transformation is surjective, or etale, or smooth, which is important, because we will define algebraic spaces and stacks as functors and categories, respectively, but we want them to still be closely related, or similar enough, to schemes.

A representable functor is a functor from \mathcal{C} to \textbf{Sets} which is naturally isomorphic to the functor which assigns to any object X the set of morphisms \text{Hom}(X,U), for some fixed object U of \mathcal{C}.

A representable presheaf is a contravariant functor from \mathcal{C} to \textbf{Sets} which is naturally isomorphic to the functor which assigns to any object X the set of morphisms \text{Hom}(U,X), for some fixed object U of \mathcal{C}. If \mathcal{C} is the category of schemes, the latter functor is also called the functor of points of the object U.

We take this opportunity to emphasize a very important concept in modern algebraic geometry. The functor of points h_{U} of a scheme U may be identified with U itself. There are many advantages to this point of view (which is also known as functorial algebraic geometry); in particular we will need it later when we give the definition of algebraic spaces and stacks.

We now have the idea of a representable functor. Next we want to have an idea of a representable natural transformation (or representable morphism) of functors. We will need another prerequisite, that of a fiber product of functors.

Let F,G,H:\mathcal{C}^{\text{op}}\rightarrow \textbf{Sets} be functors, and let a:F\rightarrow G and b:H\rightarrow G be natural transformations between these functors. Then the fiber product F\times_{a,G,b}H is a functor from \mathcal{C}^{\text{op}} to \textbf{Sets}, and is given by the formula

\displaystyle (F\times_{a,G,b}H)(X)=F(X)\times_{a_{X},G(X),b_{X}}H(X)

for any object X of \mathcal{C}.

Let F,G:\mathcal{C}^{\text{op}}\rightarrow \textbf{Sets} be functors. We say that a natural transformation a:F\rightarrow G is representable, or that F is relatively representable over G if for every U\in\text{Ob}(\mathcal{C}) and any \xi\in G(U) the functor h_{U}\times_{G}F is representable.

We now let (\text{Sch}/S)_{\text{fppf}} be the site (a category with a Grothendieck topology –  see also More Category Theory: The Grothendieck Topos) whose underlying category is the category of S-schemes, and whose coverings are given by families of flat, locally finitely presented morphisms. Any etale covering or Zariski covering is an example of this “fppf covering” (“fppf” stands for fidelement plate de presentation finie, which is French for faithfully flat and finitely presented).

An algebraic space over a scheme S is a presheaf

\displaystyle F:((\text{Sch}/S)_{\text{fppf}})^{\text{op}}\rightarrow \textbf{Sets}

with the following properties

(1) The presheaf F is a sheaf.

(2) The diagonal morphism F\rightarrow F\times F is representable.

(3) There exists a scheme U\in\text{Ob}((\text{Sch}/S)_{\text{fppf}}) and a map h_{U}\rightarrow F which is surjective, and etale (This is often written simply as U\rightarrow F). The scheme U is also called an atlas.

The diagonal morphism being representable implies that the natural transformation h_{U}\rightarrow F is also representable, and this is what allows us to describe it as surjective and etale, as has been explained earlier.

An algebraic space is a generalization of the notion of a scheme. In fact, a scheme is simply the case where, for the third condition, we have U is the disjoint union of affine schemes U_{i} and where the map h_{U}\rightarrow F is an open immersion. We recall that a scheme may be thought of as being made up of affine schemes “glued together”. This “gluing” is obtained using the Zariski topology. The notion of an algebraic space generalizes this to the etale topology.

Next we want to define algebraic stacks. Unlike algebraic spaces, which we defined as presheaves (functors), we will define algebraic stacks as categories, so we need to once again revisit the notion of representability in terms of categories.

Let \mathcal{C} be a category. A category fibered in groupoids p:\mathcal{S}\rightarrow\mathcal{C} is called representable if there exists an object X of \mathcal{C} and an equivalence j:\mathcal{S}\rightarrow \mathcal{C}/X (The notation \mathcal{C}/X signifies a slice category, whose objects are morphisms f:U\rightarrow X in \mathcal{C}, and whose morphisms are morphisms h:U\rightarrow V in \mathcal{C} such that f=g\circ h, where g:U\rightarrow X).

We give two specific special cases of interest to us (although in this post we will only need the latter):

Let \mathcal{X} be a category fibered in groupoids over (\text{Sch}/S)_{\text{fppf}}. Then \mathcal{X} is representable by a scheme if there exists a scheme U\in\text{Ob}((\text{Sch}/S)_{\text{fppf}}) and an equivalence j:\mathcal{X}\rightarrow (\text{Sch}/U)_{\text{fppf}} of categories over (\text{Sch}/S)_{\text{fppf}}.

A category fibered in groupoids p : \mathcal{X}\rightarrow (\text{Sch}/S)_{\text{fppf}} is representable by an algebraic space over S if there exists an algebraic space F over S and an equivalence j:\mathcal{X}\rightarrow \mathcal{S}_{F} of categories over (\text{Sch}/S)_{\text{fppf}}.

Next, following what we did earlier for the case of algebraic spaces, we want to define the notion of representability (by algebraic spaces) for morphisms of categories fibered in groupoids (these are simply functors satisfying some compatibility conditions with the extra structure of the category). We will need, once again, the notion of a fiber product, this time of categories over some other fixed category.

Let F:\mathcal{X}\rightarrow\mathcal{S} and G:\mathcal{Y}\rightarrow\mathcal{S} be morphisms of categories over \mathcal{C}. The fiber product \mathcal{X}\times_{\mathcal{S}}\mathcal{Y} is given by the following description:

(1) an object of \mathcal{X}\times_{\mathcal{S}}\mathcal{Y} is a quadruple (U,x,y,f), where U\in\text{Ob}(\mathcal{C}), x\in\text{Ob}(\mathcal{X}_{U}), y\in\text{Ob}(\mathcal{Y}_{U}), and f : F(x)\rightarrow G(y) is an isomorphism in \mathcal{S}_{U},

(2) a morphism (U,x,y,f) \rightarrow (U',x',y',f') is given by a pair (a,b), where a:x\rightarrow x' is a morphism in X, and b:y\rightarrow y' is a morphism in Y such that a and b induce the same morphism U\rightarrow U', and f'\circ F(a)=G(b)\circ f.

Let S be a scheme. A morphism f:\mathcal{X}\rightarrow \mathcal{Y} of categories fibered in groupoids over (\text{Sch}/S)_{\text{fppf}} is called representable by algebraic spaces if for any U\in\text{Ob}((\text{Sch}/S)_{\text{fppf}}) and any y:(\text{Sch}/U)_{\text{fppf}}\rightarrow\mathcal{Y} the category fibered in groupoids

\displaystyle (\text{Sch}/U)_{\text{fppf}}\times_{y,\mathcal{Y}}\mathcal{X}

over (\text{Sch}/U)_{\text{fppf}} is representable by an algebraic space over U.

An algebraic stack (or Artin stack) over a scheme S is a category

\displaystyle p:\mathcal{X}\rightarrow (\text{Sch}/S)_{\text{fppf}}

with the following properties:

(1) The category \mathcal{X} is a stack in groupoids over (\text{Sch}/S)_{\text{fppf}} .

(2) The diagonal \Delta:\mathcal{X}\rightarrow \mathcal{X}\times\mathcal{X} is representable by algebraic spaces.

(3) There exists a scheme U\in\text{Ob}((\text{Sch/S})_{\text{fppf}}) and a morphism (\text{Sch}/U)_{\text{fppf}}\rightarrow\mathcal{X} which is surjective and smooth (This is often written simply as U\rightarrow\mathcal{X}). Again, the scheme U is called an atlas.

If the morphism (\text{Sch}/U)_{\text{fppf}}\rightarrow\mathcal{X} is surjective and etale, we have a Deligne-Mumford stack.

Just as an algebraic space is a generalization of the notion of a scheme, an algebraic stack is also a generalization of the notion of an algebraic space (recall that that a presheaf can be thought of as category fibered in sets, which themselves are special cases of groupoids). Therefore, the definition of an algebraic stack closely resembles the definition of an algebraic space given earlier, including the requirement that the diagonal morphism (which in this case is a functor between categories) be representable, so that the functor (\text{Sch}/U)_{\text{fppf}}\rightarrow\mathcal{X} is also representable, and we can describe it as being surjective and smooth (or surjective and etale).

As an example of an application of the ideas just discussed, we mention the moduli stack of elliptic curves (which we denote by \mathcal{M}_{1,1} – the reason for this notation will become clear later). A family of elliptic curves over some “base space” B is a fibration \pi:X\rightarrow B with a section O:B\rightarrow X such that the fiber \pi^{-1}(b) over any point b of B is an elliptic curve with origin O(b).

Ideally what we want is to be able to obtain every family X\rightarrow B by pulling back a “universal object” E\rightarrow\mathcal{M}_{1,1} via the map B\rightarrow\mathcal{M}_{1,1}. This is something that even the notion of moduli space that we discussed in The Moduli Space of Elliptic Curves cannot do (we suggestively denote that moduli space by M_{1,1}). So we need the concept of stacks to construct this “moduli stack” that has this property. A more thorough discussion would need the notion of quotient stacks and orbifolds, but we only mention that the moduli stack of elliptic curves is in fact a Deligne-Mumford stack.

More generally, we can construct the moduli stack of curves of genus g with \nu marked points, denoted \mathcal{M}_{g,\nu}. The moduli stack of elliptic curves is simply the special case \mathcal{M}_{1,1}. Aside from just curves of course, we can construct moduli stacks for many more mathematical objects, such subschemes of some fixed scheme, or vector bundles, also on some fixed scheme.

The subject of algebraic stacks is a vast one, as may perhaps be inferred from the size of one of the main references for this post, the open-source reference The Stacks Project, which consists of almost 6,000 pages at the time of this writing. All that has been attempted in this post is but an extremely “bare bones” introduction to some of its more basic concepts. Hopefully more on stacks will be featured in future posts on the blog.

References:

Stack on Wikipedia

Algebraic Space on Wikipedia

Fibred Category on Wikipedia

Descent Theory on Wikipedia

Stack on nLab

Grothendieck Fibration on nLab

Algebraic Space on nLab

Algebraic Stack on nLab

Moduli Stack of Elliptic Curves on nLab

Stacks for Everybody by Barbara Fantechi

What is…a Stack? by Dan Edidin

Notes on the Construction of the Moduli Space of Curves by Dan Edidin

Notes on Grothendieck Topologies, Fibered Categories and Descent Theory by Angelo Vistoli

Lectures on Moduli Spaces of Elliptic Curves by Richard Hain

The Stacks Project

Algebraic Spaces and Stacks by Martin Olsson

Fundamental Algebraic Geometry: Grothendieck’s FGA Explained by Barbara Fantechi, Lothar Gottsche, Luc Illusie, Steven L. Kleiman, Nitin Nitsure, and Angelo Vistoli

The Theory of Motives

The theory of motives originated from the observation, sometime in the 1960’s, that in algebraic geometry there were several different cohomology theories (see Homology and Cohomology and Cohomology in Algebraic Geometry), such as Betti cohomology, de Rham cohomology, l-adic cohomology, and crystalline cohomology. The search for a “universal cohomology theory”, such that all these other cohomology theories could be obtained from such a universal cohomology theory is what led to the theory of motives.

The four cohomology theories enumerated above are examples of what is called a Weil cohomology theory. A Weil cohomology theory, denoted H^{*}, is a functor (see Category Theory) from the category \mathcal{V}(k) of smooth projective varieties over some field k to the category \textbf{GrAlg}(K) of graded K-algebras, for some other field K which must be of characteristic zero, satisfying the following axioms:

(1) (Finite-dimensionality) The homogeneous components H^{i}(X) of H^{*}(X) are finite dimensional for all i, and H^{i}(X)=0 whenever i<0 or i>2n, where n is the dimension of the smooth projective variety X.

(2) (Poincare duality) There is an orientation isomorphism H^{2n}\cong K, and a nondegenerate bilinear pairing H^{i}(X)\times H^{2n-i}(X)\rightarrow H^{2n}\cong K.

(3) (Kunneth formula) There is an isomorphism

\displaystyle H^{*}(X\times Y)\cong H^{*}(X)\otimes H^{*}(Y).

(4) (Cycle map) There is a mapping \gamma_{X}^{i} from C^{i}(X), the abelian group of algebraic cycles of codimension i on X (see Algebraic Cycles and Intersection Theory), to H^{i}(X), which is functorial with respect to pullbacks and pushforwards, has the multiplicative property \gamma_{X\times Y}^{i+j}(Z\times W)=\gamma_{X}^{i}(Z)\otimes \gamma_{Y}^{j}(W), and such that \gamma_{\text{pt}}^{i} is the inclusion \mathbb{Z}\hookrightarrow K.

(5) (Weak Lefschetz axiom) If W is a smooth hyperplane section of X, and j:W\rightarrow X is the inclusion, the induced map j^{*}:H^{i}(X)\rightarrow H^{i}(W) is an isomorphism for i\leq n-2, and a monomorphism for i\leq n-1.

(6) (Hard Lefschetz axiom) The Lefschetz operator

\displaystyle \mathcal{L}:H^{i}(X)\rightarrow H^{i+2}(X)

given by

\displaystyle \mathcal{L}(x)=x\cdot\gamma_{X}^{1}(W)

for some smooth hyperplane section W of X, with the product \cdot provided by the graded K-algebra structure of H^{*}(X), induces an isomorphism

\displaystyle \mathcal{L}^{i}:H^{n-i}(X)\rightarrow H^{n+i}(X).

The idea behind the theory of motives is that all Weil cohomology theories should factor through a “category of motives”, i.e. any Weil cohomology theory

\displaystyle H^{*}: \mathcal{V}(k)\rightarrow \textbf{GrAlg}(K)

can be expressed as the following composition of functors:

\displaystyle H^{*}: \mathcal{V}(k)\xrightarrow{h} \mathcal{M}(k)\rightarrow\textbf{GrAlg}(K)

where \mathcal{M}(k) is the category of motives. We can get different Weil cohomology theories, such as Betti cohomology, de Rham cohomology, l-adic cohomology, and crystalline cohomology, via different functors (called realization functors) from the category of motives to a category of graded algebras over some field K. This explains the term “motive”, which actually comes from the French word “motif”, which itself is already used in music and visual arts, among other things, as some kind of common underlying “theme” with different possible manifestations.

Let us now try to construct this category of motives. This category is often referred to in the literature as a “linearization” of the category of smooth projective varieties. This means that we obtain it from some sense starting with the category of smooth projective varieties, but we also want to modify it so that it we can do linear algebra, or more properly homological algebra, in some sense. In other words, we want it to behave like the category of modules over some ring. With this in mind, we want the category to be an abelian category, so that we can make sense of notions such as kernels, cokernels, and exact sequences.

An abelian category is a category that satisfies the following properties:

(1) The morphisms form an abelian group.

(2) There is a zero object.

(3) There are finite products and coproducts.

(4) Every morphism f:X\rightarrow Y has a kernel and cokernel, and satisfies a decomposition

\displaystyle K\xrightarrow{k} X\xrightarrow{i} I\xrightarrow{j} Y\xrightarrow{c} K'

where K is the kernel of f, K' is the cokernel of f, and I is the kernel of c and the cokernel of k (not to be confused with our notation for fields).

In order to proceed with our construction of the category of motives, which we now know we want to be an abelian category, we discuss the notion of correspondences.

The group of correspondences of degree r from a smooth projective variety X to another smooth projective variety Y, written \text{Corr}^{r}(X,Y), is defined to be the group of algebraic cycles of X\times Y of codimension n+r, where n is the dimension of X, i.e.

\text{Corr}^{r}(X,Y)=C^{n+r}(X\times Y)

A morphism (of varieties, in the usual sense) f:Y\rightarrow X determines a correspondence from X to Y of degree 0 given by the transpose of the graph of f in X\times Y. Therefore we may think of correspondences as generalizations of the usual concept of morphisms of varieties.

As we have learned in Algebraic Cycles and Intersection Theory, whenever we are dealing with algebraic cycles, it is often useful to consider them only up to some equivalence relation. In the aforementioned post we introduced the notion of rational equivalence. This time we consider also homological equivalence and numerical equivalence between algebraic cycles.

We say that two algebraic cycles Z_{1} and Z_{2} are homologically equivalent if they have the same image under the cycle map, and we say that they are numerically equivalent if the intersection numbers Z_{1}\cdot Z and Z_{2}\cdot Z are equal for all Z of complementary dimension. There are other such equivalence relations on algebraic cycles, but in this post we will only mostly be using rational equivalence, homological equivalence, and numerical equivalence.

Since correspondences are algebraic cycles, we often consider them only up to these equivalence relations, and denote the quotient group we obtain by \text{Corr}_{\sim}^{r}(X,Y), where \sim is the equivalence relation imposed, for example, for numerical equivalence we write \text{Corr}_{\text{num}}^{r}(X,Y).

Taking the tensor product of the abelian group \text{Corr}_{\sim}^{r}(X,Y) with the rational numbers \mathbb{Q}, we obtain the vector space

\displaystyle \text{Corr}_{\sim}^{r}(X,Y)_{\mathbb{Q}}=\text{Corr}_{\sim}^{r}(X,Y)\otimes_{\mathbb{Z}}\mathbb{Q}

To obtain something closer to an abelian category (more precisely, we will obtain what is known as a pseudo-abelian category, but in the case where the equivalence relation is numerical equivalence, we will actually obtain an abelian category), we need to consider “projectors”, correspondences p of degree 0 from a variety X to itself such that p^{2}=p. So now we form a category, whose objects are h(X,p) for a variety X and projector p, and whose morphisms are given by

\displaystyle \text{Hom}(h(X,p),h(Y,q))=q\circ\text{Corr}_{\sim}^{0}(X,Y)_{\mathbb{Q}}\circ p.

We call this category the category of pure effective motives, and denote it by \mathcal{M}_{\sim}^{\text{eff}}(k). The process described above is also known as passing to the pseudo-abelian (or Karoubian) envelope.

We write h^{i}(X,p) for the objects of \mathcal{M}_{\sim}^{\text{eff}}(k) that map to H^{i}(X). In the case that X is the projective line \mathbb{P}^{1}, and p is the diagonal \Delta_{\mathbb{P}^{1}}, we find that

h(\mathbb{P}^{1},\Delta_{\mathbb{P}^{1}})=h^{0}\mathbb{P}^{1}\oplus h^{2}\mathbb{P}^{1}

which can be rewritten also as

\displaystyle h(\mathbb{P}^{1},\Delta_{\mathbb{P}^{1}})=\mathbb{I}\oplus\mathbb{L}

where \mathbb{I} is the image of a point in the category of pure effective motives, and \mathbb{L} is known as the Lefschetz motive. It is also denoted by \mathbb{Q}(-1). The above decomposition corresponds to the projective line \mathbb{P}^{1} being a union of the affine line \mathbb{A}^{1} and a “point at infinity”, which we may denote by \mathbb{A}^{0}:

\displaystyle \mathbb{P}^{1}=\mathbb{A}^{0}\cup\mathbb{A}^{1}

More generally, we have

\displaystyle h(\mathbb{P}^{n},\Delta_{\mathbb{P}^{n}})=\mathbb{I}\oplus\mathbb{L}\oplus...\oplus\mathbb{L}^{n}

corresponding to

\displaystyle \mathbb{P}^{n}=\mathbb{A}^{0}\cup\mathbb{A}^{1}\cup...\cup\mathbb{A}^{n}.

The category of effective pure motives is an example of a tensor category. This means it has a bifunctor \otimes: \mathcal{M}_{\sim}^{\text{eff}}\times\mathcal{M}_{\sim}^{\text{eff}}\rightarrow\mathcal{M}_{\sim}^{\text{eff}} which generalizes the usual notion of a tensor product, and in this particular case it is given by taking the product of two varieties. We can ask for more, however, and construct a category of motives which is not just a tensor category but a rigid tensor category, which provides us with a notion of duals.

By formally inverting the Lefschetz motive (the formal inverse of the Lefschetz motive is then known as the Tate motive, and is denoted by \mathbb{Q}(1)), we can obtain this rigid tensor category, whose objects are triples h(X,p,m), where X is a variety, e is a projector, and m is an integer. The morphisms of this category are given by

\displaystyle \text{Hom}(h(X,p,n),h(Y,q,m))=q\circ\text{Corr}_{\sim}^{n-m}(X,Y)_{\mathbb{Q}}\circ p.

This category is called the category of pure motives, and is denoted by \mathcal{M}_{\sim}(k). The category \mathcal{M}_{\text{rat}}(k) is called the category of Chow motives, while the category \mathcal{M}_{\text{num}}(k) is called the category of Grothendieck (or numerical) motives.

The category of Chow motives has the advantage that it is known to be “universal”, in the sense that every Weil cohomology theory factors through it, as discussed earlier; however, in general it is not even abelian, which is a desirable property we would like our category of motives to have. Meanwhile, the category of Grothendieck motives is known to be abelian, but it is not yet known if it is universal. If the so-called “standard conjectures on algebraic cycles“, which we will enumerate below, are proved, then the category of Grothendieck motives will be known to be universal.

We have seen that the category of pure motives forms a rigid tensor category. Closely related to this concept, and of interest to us, is the notion of a Tannakian category. More precisely, a Tannakian category is a k-linear rigid tensor category with an exact faithful functor (called a fiber functor) to the category of finite-dimensional vector spaces over some field extension K of k.

One of the things that makes Tannakian categories interesting is that there is an equivalence of categories between a Tannakian category \mathcal{C} and the category \text{Rep}_{G} of finite-dimensional linear representations of the group of automorphisms of its fiber functor, which is also known as the Tannakian Galois group, or, if the Tannakian category is a “category of motives” of some sort, the motivic Galois group. This aspect of Tannakian categories may be thought of as a higher-dimensional analogue of the classical theory of Galois groups, which can be stated as an equivalence of categories between the category of finite separable field extensions of a field k and the category of finite sets equipped with an action of the Galois group \text{Gal}(\bar{k}/k), where \bar{k} is the algebraic closure of k.

So we see that being a Tannakian category is yet another desirable property that we would like our category of motives to have. For this not only do we have to tweak the tensor product structure of our category, we also need certain conjectural properties to hold. These are the same conjectures we have hinted at earlier, called the standard conjectures on algebraic cycles, formulated by Alexander Grothendieck at around the same time he initially developed the theory of motives.

These conjectures have some very important consequences in algebraic geometry, and while they remain unproved to this day, the search for their proof (or disproof) is an important part of modern mathematical research on the theory of motives. They are the following:

(A) (Standard conjecture of Lefschetz type) For i\leq n, the operator \Lambda defined by

\displaystyle \Lambda=(\mathcal{L}^{n-i+2})^{-1}\circ\mathcal{L}\circ (\mathcal{L}^{n-i}):H^{i}\rightarrow H^{i-2}

\displaystyle \Lambda=(\mathcal{L}^{n-i})\circ\mathcal{L}\circ (\mathcal{L}^{n-i+2})^{-1}:H^{2n-i+2}\rightarrow H^{2n-i}

is induced by algebraic cycles.

(B) (Standard conjecture of Hodge type) For all i\leq n/2, the pairing

\displaystyle x,y\mapsto (-1)^{i}(\mathcal{L}x\cdot y)

is positive definite.

(C) (Standard conjecture of Kunneth type) The projectors H^{*}(X)\rightarrow H^{i}(X) are induced by algebraic cycles in X\times X with rational coefficients. This implies the following decomposition of the diagonal:

\displaystyle \Delta_{X}=\pi_{0}+...+\pi_{2n}

which in turn implies the decomposition

\displaystyle h(X,\Delta_{X},0)=h(X,\pi_{0},0)\oplus...\oplus h(X,\pi_{2n},0)

which, writing h(X,\Delta_{X},0) as hX and h(X,\pi_{i},0) as h^{i}(X), we can also compactly and suggestively write as

\displaystyle hX=h^{0}X\oplus...\oplus h^{2n}X.

In other words, every object hX=h(X,\Delta_{X},0) of our “category of motives” decomposes into graded “pieces” h^{i}(X)=h(X,\pi_{i},0) of pure “weighti. We have already seen earlier that this is indeed the case when X=\mathbb{P}^{n}. We will need this conjecture to hold if we want our category to be a Tannakian category.

(D) (Standard conjecture on numerical equivalence and homological equivalence) If an algebraic cycle is numerically equivalent to zero, then its cohomology class is zero. If the category of Grothendieck motives is to be “universal”, so that every Weil cohomology theory factors through it, this conjecture must be satisfied.

In Algebraic Cycles and Intersection Theory and Some Useful Links on the Hodge Conjecture, Kahler Manifolds, and Complex Algebraic Geometry, we have made mention of the two famous conjectures in algebraic geometry known as the Hodge conjecture and the Tate conjecture. In fact, these two closely related conjectures can be phrased in the language of motives as the conjectures stating that the realization functors from the category of motives to the category of pure Hodge structures and continuous l-adic representations of \text{Gal}(\bar{k}/k), respectively, be fully faithful. These conjectures are closely related to the standard conjectures on algebraic cycles as well.

We have now constructed the category of pure motives, for smooth projective varieties. For more general varieties and schemes, there is an analogous idea of “mixed motives“, which at the moment remain conjectural, although there exist several related constructions which are the closest thing we currently have to such a theory of mixed motives.

If we want to construct a theory of mixed motives, instead of Weil cohomology theories we must instead consider what are known as “mixed Weil cohomology theories“, which are expected to have the following properties:

(1) (Homotopy invariance) The projection \pi:X\rightarrow\mathbb{A}^{1} induces an isomorphism

\displaystyle \pi^{*}:H^{*}(X)\xrightarrow{\cong}H^{*}(X\times\mathbb{A}^{1})

(2) (Mayer-Vietoris sequence) If U and V are open coverings of X, then there is a long exact sequence

\displaystyle ...\rightarrow H^{i}(U\cap V)\rightarrow H^{i}(X)\rightarrow H^{i}(U)\oplus H^{i}(V)\rightarrow H^{i}(U\cap V)\rightarrow...

(3) (Duality) There is a duality between cohomology H^{*} and cohomology with compact support H_{c}^{*}.

(4) (Kunneth formula) This is the same axiom as the one in the case of pure motives.

We would like a category of mixed motives, which serves as an analogue to the category of pure motives in that all mixed Weil cohomology theories factor through it, but as mentioned earlier, no such category exists at the moment. However, the mathematicians Annette Huber-Klawitter, Masaki Hanamura, Marc Levine, and Vladimir Voevodsky have constructed different versions of a triangulated category of mixed motives, denoted \mathcal{DM}(k).

A triangulated category \mathcal{T} is an additive category with an automorphism T: \mathcal{T}\rightarrow\mathcal{T} called the “shift functor” (we will also denote T(X) by X[1], and T^{n}(X) by X[n], for n\in\mathbb{Z}) and a family of “distinguished triangles

\displaystyle X\rightarrow Y\rightarrow Z\rightarrow X[1]

 which satisfies the following axioms:

(1) For any object X of \mathcal{T}, the triangle X\xrightarrow{\text{id}}X\rightarrow 0\rightarrow X[1] is a distinguished triangle.

(2) For any morphism u:X\rightarrow Y of \mathcal{T}, there is an object Z of \mathcal{T} such that X\xrightarrow{u}Y\rightarrow Z\rightarrow X[1] is a distinguished triangle.

(3) Any triangle isomorphic to a distinguished triangle is a distinguished triangle.

(4) If X\rightarrow Y\rightarrow Z\rightarrow X[1] is a distinguished triangle, then the two “rotations” Y\rightarrow Z\rightarrow Z[1]\rightarrow Y[1] and Z[-1]\rightarrow X\rightarrow Y\rightarrow Z are also distinguished triangles.

(5) Given two distinguished triangles X\xrightarrow{u}Y\xrightarrow{v}Z\xrightarrow{w}X[1] and X'\xrightarrow{u'}Y'\xrightarrow{v'}Z'\xrightarrow{w'}X'[1] and morphisms f:X\rightarrow X' an g:Y\rightarrow Y' such that the square “commutes”, i.e. u'\circ f=g\circ u, there exists a morphisms h:Z\rightarrow Z such that all other squares commute.

(6) Given three distinguished triangles X\xrightarrow{u}Y\xrightarrow{j}Z'\xrightarrow{k}X[1]Y\xrightarrow{v}Z\xrightarrow{l}X'\xrightarrow{i}Y[1], and X\xrightarrow{v\circ u}Z\xrightarrow{m}Y'\xrightarrow{n}X[1], there exists a distinguished triangle Z'\xrightarrow{f}Y'\xrightarrow{g}X'\xrightarrow{h}Z'[1] such that “everything commutes”.

A t-structure on a triangulated category \mathcal{T} is made up of two full subcategories \mathcal{T}^{\geq 0} and \mathcal{T}^{\leq 0} satisfying the following properties (writing \mathcal{T}^{\leq n} and \mathcal{T}^{\leq n} to denote \mathcal{T}^{\leq 0}[-n] and \mathcal{T}^{\geq 0}[-n] respectively):

(1) \mathcal{T}^{\leq -1}\subset \mathcal{T}^{\leq 0} and \mathcal{T}^{\geq 1}\subset \mathcal{T}^{\geq 0}

(2) \displaystyle \text{Hom}(X,Y)=0 for any object X of \mathcal{T}^{\leq 0} and any object Y of \mathcal{T}^{\geq 1}

(3) for any object Y of \mathcal{T} we have a distinguished triangle

\displaystyle X\rightarrow Y\rightarrow Z\rightarrow X[1]

where X is an object of \mathcal{T}^{\leq 0} and Z is an object of \mathcal{T}^{\geq 1}.

The full subcategory \mathcal{T}^{0}=\mathcal{T}^{\leq 0}\cap\mathcal{T}^{\geq 0} is called the heart of the t-structure, and it is an abelian category.

It is conjectured that the category of mixed motives \mathcal{MM}(k) is the heart of the t-structure of the triangulated category of mixed motives \mathcal{DM}(k).

Voevodsky’s construction proceeds in a manner somewhat analogous to the construction of the category of pure motives as above, starting with schemes (say, over a field k, although a more general scheme may be used) as objects and correspondences as morphisms, but then makes use of concepts from abstract homotopy theory, such as taking the bounded homotopy category of bounded complexes, and localization with respect to a certain subcategory, before passing to the pseudo-abelian envelope and then formally inverting the Tate object \mathbb{Z}(1). The triangulated category obtained is called the category of geometric motives, and is denoted by \mathcal{DM}_{\text{gm}}(k). The schemes and correspondences involved in the construction of \mathcal{DM}_{\text{gm}}(k) are required to satisfy certain properties which eliminates the need to consider the equivalence relations which form a large part of the study of the category of pure motives.

Closely related to the triangulated category of mixed motives is motivic cohomology, which is defined in terms of the former as

\displaystyle H^{i}(X,\mathbb{Z}(m))=\text{Hom}_{\mathcal{DM}(k)}(X,\mathbb{Z}(m)[i])

where \mathbb{Z}(m) is the tensor product of m copies of the Tate object \mathbb{Z}(1), and the notation \mathbb{Z}(m)[i] tells us that the shift functor of the triangulated category is applied to the object \mathbb{Z}(m) i times.

Motivic cohomology is related to the Chow group, which we have introduced in Algebraic Cycles and Intersection Theory, and also to algebraic K-theory, which is another way by which the ideas of homotopy theory are applied to more general areas of abstract algebra and linear algebra. These ideas were used by Voevodsky to prove several related theorems, from the Milnor conjecture to its generalization, the Bloch-Kato conjecture (also known as the norm residue isomorphism theorem).

Historically, one of the motivations for Grothendieck’s attempt to obtain a universal cohomology theory was to prove the Weil conjectures, which is a higher-dimensional analogue of the Riemann hypothesis for curves over finite fields first proved by Andre Weil himself (see The Riemann Hypothesis for Curves over Finite Fields). In fact, if the standard conjectures on algebraic cycles are proved, then a proof of the Weil conjectures would follow via an approach that closely mirrors Weil’s original proof (since cohomology provides a Lefschetz fixed-point formula –  we have mentioned in The Riemann Hypothesis for Curves over Finite Fields that the study of fixed points is an important part of Weil’s proof). The last of the Weil conjectures were eventually proved by Grothendieck’s student Pierre Deligne, but via a different approach that bypassed the standard conjectures. A proof of the standard conjectures, which would lead to a perhaps more elegant proof of the Weil conjectures, is still being pursued to this day.

The theory of motives is not only related to analogues of the Riemann hypothesis, which concerns the location of zeroes of L-functions, but to L-functions in general. For instance, it is also related to the Langlands program, which concerns another aspect of L-functions, namely their analytic continuation and functional equation, and to the Birch and Swinnerton-Dyer conjecture, which concerns their values at special points.

We recall in The Riemann Hypothesis for Curves over Finite Fields that the Frobenius morphism played an important part in counting the points of a curve over a finite field, which in turn we needed to define the zeta function (of which the L-function can be thought of as a generalization) of the curve. The Frobenius morphism is an element of the Galois group, and we recall that a category of motives which is a Tannakian category is equivalent to the category of representations of its motivic Galois group. Therefore we can see how we can define “motivic L-functions” using the theory of motives.

As the L-functions occupy a central place in many areas of modern mathematics, the theory of motives promises much to be gained from its study, if only we could make progress in deciphering the many mysteries that surround it, of which we have only scratched the surface in this post. The applications of motives are not limited to L-functions either – the study of periods, which relate Betti cohomology and de Rham cohomology, and lead to transcendental numbers which can be defined using only algebraic concepts, is also strongly connected to the theory of motives. Recent work by the mathematicians Alain Connes and Matilde Marcolli has also suggested applications to physics, particularly in relation to Feynman diagrams in quantum field theory. There is also another generalization of the theory of motives, developed by Maxim Kontsevich, in the context of noncommutative geometry.

References:

Weil Cohomology Theory on Wikipedia

Motive on Wikipedia

Standard Conjectures on Algebraic Cycles on Wikipedia

Motive on nLab

Pure Motive on nLab

Mixed Motive on nLab

The Tate Conjecture over Finite Fields on Hard Arithmetic

What is…a Motive? by Barry Mazur

Motives – Grothendieck’s Dream by James S. Milne

Noncommutative Geometry, Quantum Fields, and Motives by Alain Connes and Matilde Marcolli

Algebraic Cycles and the Weil Conjectures by Steven L. Kleiman

The Standard Conjectures by Steven L. Kleiman

Feynman Motives by Matilde Marcolli

Une Introduction aux Motifs (Motifs Purs, Motifs Mixtes, Periodes) by Yves Andre

Basics of Math and Physics

I’ve added another new page to the blog, called Basics of Math and Physics. As most of the posts lately have tackled subjects of increasing technical sophistication, I thought it would be a good idea to collect some of the more introductory posts in one page. It is not an exhaustive list, however, as this blog has not tackled all the basic subjects in math and physics; for example, there are no posts yet discussing real analysis or complex analysis. Perhaps if these subjects are tackled in future posts they will be added to that page too. It is also best supplemented with the references listed in Book List.

Some Useful Links on the History of Algebraic Geometry

It’s been a while since I’ve posted on this blog, but there are some posts I’m currently working on about some subjects I’m also currently studying (that’s why it’s taking so long, as I’m trying to digest the ideas as much as I can before I can post about it). But anyway, for the moment, in this short post I’ll be putting up some links to articles on the history of algebraic geometry. Aside from telling an interesting story on its own, there is also much to be learned about a subject from studying its historical development.

We know that the origins of algebraic geometry can be traced back to Rene Descartes and Pierre de Fermat in the 17th century. This is the high school subject also known as “analytic geometry” (which, as we have mentioned in Basics of Algebraic Geometry, can be some rather confusing terminology, because in modern times the word “analytic” is usually used to refer to concepts in complex calculus).

The so-called “analytic geometry” seems to be a rather straightforward subject compared to modern-day algebraic geometry, which, as may be seen on many of the previous posts on this blog, is very abstract (but it is also this abstraction that gives it its power). How did this transformation come to be?

The mathematician Jean Dieudonne, while perhaps more known for his work in the branch of mathematics we call analysis (the more high-powered version of calculus), also served as adviser to Alexander Grothendieck, one of the most important names in the development of modern algebraic geometry. Together they wrote the influential work known as Elements de Geometrie Algebrique, often simply referred to as EGA. Dieudonne was also among the founding members of the “Bourbaki group”, a group of mathematicians who greatly influenced the development of modern mathematics. Himself a part of its development, Dieudonne wrote many works on the history of mathematics, among them the following article on the history of algebraic geometry which can be read for free on the website of the Mathematical Association of America:

The Historical Development of Algebraic Geometry by Jean Dieudonne

But before the sweeping developments instituted by Alexander Grothendieck, the modern revolution in algebraic geometry was first started by the mathematicians Oscar Zariski and Andre Weil (we discussed some of Weil’s work in The Riemann Hypothesis for Curves over Finite Fields). Zariski himself learned from the so-called “Italian school of algebraic geometry”, particularly the mathematicians Guido Castelnuovo, Federigo Enriques, and Francesco Severi.

At the International Congress of Mathematicians in 1950, both Zariski and Weil presented, separately, a survey of the developments in algebraic geometry at the time, explaining how the new “abstract algebraic geometry” was different from the old “classical algebraic geometry”, and the new advantages it presented. The proceedings of this conference are available for free online:

Proceedings of the 1950 International Congress of Mathematicians, Volume I

Proceedings of the 1950 International Congress of Mathematicians, Volume II

The articles by Weil and Zariski can be found in the second volume, but I included also the first volume for “completeness”.

All proceedings of the International Congress of Mathematicians, which is held every four years, are actually available for free online:

Proceedings of the International Congress of Mathematicians, 1983-2010

The proceedings of the 2014 International Congress of Mathematicians in Seoul, Korea, can be found here:

Proceedings of the 2014 International Congress of Mathematicians

Going back to algebraic geometry, a relatively easy to understand (for those with some basic mathematical background, anyway) summary of the work of Alexander Grothendieck’s work in algebraic geometry can be found in the following article by Colin McLarty, published in April 2016 issue of the Notices of the American Mathematical Society:

How Grothendieck Simplified Algebraic Geometry by Colin McLarty

Tangent Spaces in Algebraic Geometry

We have discussed the notion of a tangent space in Differentiable Manifolds Revisited in the context of differential geometry. In this post we take on the same topic, but this time in the context of algebraic geometry, where it is also known as the Zariski tangent space (when no confusion arises, however, it is often simply referred to as the tangent space).

This will present us with challenges, since the concept of the tangent space is perhaps best tackled using the methods of calculus, but in algebraic geometry, we want to have a notion of tangent spaces in cases where we would not usually think of calculus as being applicable, for instance in the case of varieties over finite fields. In other words, we want our treatment to be algebraic. Nevertheless, we will use the methods of calculus as an inspiration.

We don’t want to be too dependent on the parts of calculus that make use of properties of the real and complex numbers that will not carry over to the more general cases. Fortunately, if we are dealing with polynomials, we can just “borrow” the “power rule” of calculus, since that “rule” only makes use of algebraic procedures, and we need not make use of sequences, limits, and so on. Namely, if we have a polynomial given by

\displaystyle f=\sum_{j=1}^{n}ax^{j}

We set

\displaystyle \frac{\partial f}{\partial x}=\sum_{j=1}^{n}jax^{j-1}

We recall the rules for partial derivatives – in the case that we are differentiating over some variable x, we simply treat all the other variables as constants, and follow the usual rules of differential calculus. With these rules, we can now make the definition of the tangent space at the point P with coordinates (a_{1},a_{2},...,a_{n}) as the algebraic set which satisfies the equation

\displaystyle \sum_{j}\frac{\partial f}{\partial x_{j}}(P)(x_{j}-a_{j})=0

For example, consider the parabola given by the equation y-x^{2}=0. Let us take the tangent space at the point P with coordinates x=1, y=1. The procedure above gives us

\displaystyle \frac{\partial f}{\partial x}(P)(x-1)+\frac{\partial f}{\partial y}(P)(y-1)=0

Since

\displaystyle \frac{\partial f}{\partial x}=-2x

\displaystyle \frac{\partial f}{\partial y}=1

We then have

\displaystyle -2x|_{x=1,y=1}(x-1)+1|_{x=1,y=1}(y-1)=0

\displaystyle -2(1)(x-1)+1(y-1)=0

\displaystyle -2x+2+y-1=0

\displaystyle y-2x+1=0

The parabola is graphed (its real part, at least, using the Desmos graphing calculator) in the diagram below in red, with its tangent space, a line, in blue:

desmos-graph

In case the reader is not convinced by our “borrowing” of concepts from calculus and claiming that they are “algebraic” in the specific case we are dealing with, another way to look at things without making reference to calculus is the following procedure, which comes from basic high school-level “analytic geometry”. First we translate the coordinate system so that the origin is at the point P where we want to take the tangent space. Then we simply take the “linear part” of the polynomial equation, then translate again so that the origin is where it used to be originally. This gives the same results as the earlier procedure (the technical justification is given by the theory of Taylor series). More explicitly we have:

\displaystyle y-x^{2}=0

Translating the origin of coordinates to the point x=1, y=1, we have

\displaystyle (y+1)-(x+1)^{2}=0

\displaystyle y+1-(x^{2}+2x+1)=0

\displaystyle y+1-x^{2}-2x-1=0

\displaystyle y-x^{2}-2x=0

We take only the linear part, which is

\displaystyle y-2x=0

And then we translate the origin of coordinates back to the original one:

\displaystyle (y-1)-2(x-1)=0

\displaystyle y-1-2x+2=0

\displaystyle y-2x+1=0

which is the same result we had earlier.

But it may happen that the polynomial has no “linear part”. In this case the tangent space is the entirety of the ambient space. However, there is another related concept which may be useful in these cases, called the tangent cone. The tangent cone is the algebraic set which satisfies the equations we get by extracting the lowest degree part of the polynomial, which may or may not be the linear part. In the case that the lowest degree part is the linear part, the tangent space and the tangent cone coincide, and if this holds for all points of a variety, we say that the variety is nonsingular.

To give an explicit example, consider the curve y^{2}=x^{3}+x^{2}, as seen in the diagram below in red (its real part graphed once again using the Desmos graphing calculator):

desmos-graph (2)

The equation that defines this curve has no linear part. Therefore the tangent space at the origin consists of all x and y which satisfy the trivial equation 0=0; but then, all values of x and y satisfy this equation, and therefore the tangent space is the “affine plane” \mathbb{A}^{2}. However, the lowest order part is y^{2}=x^{2}, which is satisfied by all points which also satisfy either of the two equations y=x or y=-x. These points form the blue and orange diagonal lines in the diagram. Since the tangent space and the tangent cone do not agree, the curve is singular at the origin.

We can also define the tangent space in a more abstract manner, using the concepts we have discussed in Localization. Let \mathfrak{m} be the unique maximal ideal of the local ring O_{X,P}, and let \mathfrak{m}^{2} be the product ideal whose elements are the sums of products of elements of \mathfrak{m}. The quotient \mathfrak{m}/\mathfrak{m}^{2} is then a vector space over the residue field k. The tangent space of X at P is then defined as the dual of this vector space (the vector space of linear transformations from \mathfrak{m}/\mathfrak{m}^{2} to k). The vector space \mathfrak{m}/\mathfrak{m}^{2} itself is called the cotangent space of X at P. We can think of its elements as linear polynomial functions on the tangent space. There is an analogous abstract definition of the tangent cone, namely as the spectrum of the graded ring \oplus_{i\geq 0}\mathfrak{m}^{i}/\mathfrak{m}^{i+1}.

References:

Zariski Tangent Space on Wikipedia

Tangent Cone on Wikipedia

Desmos Graphing Calculator

Algebraic Geometry by J.S. Milne

Algebraic Geometry by Andreas Gathmann

Algebraic Geometry by Robin Hartshorne