Notes on divisors-2

Weil divisors v.s. Cartier divisors

Let’s continue the discussion of the last post. In this post, we shall establish a correspondence between Weil divisors and Cartier divisors for special schemes.

First let’s repeat what this correspondence is:

Suppose that X is an integral, separated noetherian scheme such that all of its local rings are UFDs. Then the group Div(X) of Weil divisors on X is isomorphic to the group \Gamma(X,\mathfrak{K}^*/\mathfrak{O}_X^*) of Cartier divisors. Besides, the principal Weil divisors correspond to the principal Cartier divisors.

This correspondence and the following proof are taken from Hartshorne’s book, ‘algebraic geometry’.

We shall not give a complete and rigorous proof to this statement. We only give a rough idea of what this correspondence is like. First note that X is integral(reduced and irreducible), thus the sheaf \mathfrak{K} is the constant sheaf of K on X where K is the function field on X, which is also the local ring \mathfrak{O}_{X,x_0} with x_0 the generic point of X. So, a Cartier divisor is given by a family \{U_i,f_i\} where U_i is an open cover of X and f_i\in \Gamma(U_i,\mathfrak{O}_X)=K^*. We now construct a Weil divisor: for each prime divisor Y of X, by definition, this means that Y is an integral closed subscheme of X, now for each f_i, we set the coefficient of Y to be v_Y(f_i). Note that for any two U_i,U_j such that U_i\bigcap U_j, then by definition, f_i/f_j\in \Gamma(U_i\bigcap U_j,\mathfrak{O}_X), that is to say, f_i/f_j is invertible on U_i\bigcap U_j, thus v_Y(f_i)=v_Y(f_j), so the coefficient of Y is well-defined. And we set the Weil divisor to be D=\sum_Yv_Y(f_i). Note here we should guarantee that this is a finite sum. This can be deduced easily from the assumption that X is noetherian(we will say about this later).

Now conversely, if D is a Weil divisor on X, then for each point x\in X, D gives a divisor on Spec(\mathfrak{O}_{X,x}). Now by assumption, \mathfrak{O}_{X,x} is a UFD, we can show that D_x is a principal divisor(we shall not prove this point for the present), that is, D_x=(f_x) for some f_x\in K^*. This means that there is an open subset x\in U of X such that f_x and D agree on U. In this way, varying x, we get an open cover U_i of X with f_i such that f_i and D agree on U_i. And we set \{U_i,f_i\} to be the corresponding Cartier divisor. So, we finish the construction of the correspondence between Weil divisors and Cartier divisors.

It is not too hard to see that these two construction are inverse one to another.

Relation to invertible sheaves

Next we want to relate divisors to invertible sheaves.

Suppose that X is a scheme and \mathfrak{O}_X its structure sheaf. Then we define an invertible sheaf to be an \mathfrak{O}_X-module locally free of rank 1.

Now let D be a Cartier divisor on X given by \{U_i,f_i\}, then we define the sheaf \mathfrak{L}(D) to be the submodule of \mathfrak{K} generated by 1/f_i on each U_i for all i. We call \mathfrak{L}(D) to be the sheaf associated to D.

The important result is that, this gives a one-to-one correspondence between the set of Cartier divisors and the set of invertible submodules of \mathfrak{K}. This can be seen like this: given a invertible submodule \mathfrak{L} of \mathfrak{K}, for an open subset U of X, \mathfrak{L}(U) is a free module of \mathfrak{O}_X(U) of rank 1, which is also a submodule of \mathfrak{K}(U). So, we take a generator f_U of \mathfrak{L}(U)(it is not zero since \mathfrak{L}(U) is of rank 1, not 0), then we get an open covering U_i of X such that on each U_i there is an f_i. We can verify that these f_i can glue together, and thus we can set \{u_i,f_i\} to be the Cartier divisor. It can be verified that this process is converse to the above one, thus we showed that there is a correspondence between the set of Cartier divisors and the set of invertible submodules of \mathfrak{K}.

In fact, this is more than a bijection, this is a group homomorphism. Recall that we give a group structure to the set of invertible sheaves on X by tensor product: suppose that \mathfrak{L},\mathfrak{L}' are two invertible sheaves on X, then by definition, \mathfrak{L}\bigotimes_{\mathfrak{O}_X}\mathfrak{L}' is again an invertible sheaf on X. Besides, \mathfrak{L}^*=Hom_{\mathfrak{O}_X}(\mathfrak{L},\mathfrak{O}_X) is also an invertible sheaf on X, and \mathfrak{L}\bigotimes \mathfrak{L}^*\simeq\mathfrak{O}_X. We call this group the Picard group of X,

Charles Emile Picard

Charles Emile Picard(photo taken from here)

and denote it by Pic(X). So the set of invertible submodules of \mathfrak{K} is a subgroup of Pic(X).

It is not hard to see that \mathfrak{L}(D/D')=\mathfrak{L}(D)\bigotimes\mathfrak{L}(D')^*, so this gives a group homomorphism between the group of Cartier divisors and the group of invertible submodules of \mathfrak{K}.

In general, the group of Cartier divisors is not equal to the Picard group of X. Yet, under some mild condition, we can show that, these two groups are isomorphic under the above morphism:

Suppose that X is an integral scheme, then the group of Cartier divisors on X is isomorphic to Pic(X).

We will establish this isomorphism in the next post.

Notes on divisors-1

Introduction

Divisors are a very important tool in algebraic geometry. Let’s quote an example from Hartshorne’s ‘Algebraic Geometry’: let C be a non-singular projective curve in the projective plane X=\mathbb{P}^2_k over a field k. Now let L be any line in X, and we write L\bigcap C for their intersection, which consists of finitely many points counted with multiplicity. Now let’s vary the line L, then we obtain a family of finite sets L\bigcap C. It is not hard to see that we can construct the embedding of C in X using this family of sets L\bigcap C. This is a very powerful tool in studying this kind of embeddings.

Weil divisors

The simplest kind of divisors is perhaps the Weil divisors.

André Weil

André Weil(photo from here)

Suppose that X is a scheme, we say that X is regular of co-dimensional one if all its local rings \mathfrak{O}_{X,x} of dimensional 1 is regular.

Let me explain these terms. Let R be a ring, then for any prime ideal \mathfrak{p} of R, its height h(\mathfrak{p}) is defined to be the largest integer n such that there exists a strictly increasing sequence of prime ideals \mathfrak{p}_0\subset \mathfrak{p}_1\subset...\subset\mathfrak{p}_n=\mathfrak{p}. And the dimension dim(R) of R is defined to be the maximum of heights of its prime ideals. This is the so-called Krull dimension of R. For some rings R, this dim(R) can be infinite, even if R is a noetherian ring, dim(R) can still be infinite, the classical examples are perhaps due to Nagata. Let k be a field and consider R=k[x_1,x_2,...,x_n,...] the polynomial ring on infinite variables. We set the prime ideals to be \mathfrak{p}_n=R<x_{n^2+1},x_{n^2+1},...,x_{(n+1)^2}> generated by the variables in the bracket. And we set S=R-\bigcup_n \mathfrak{p}_n, a multiplicative closed subset of R, then we take the localization R'=S^{-1}R. It can be shown that the maximal ideals of R' are of the form \mathfrak{p}_nR', so we have that h(\mathfrak{p}_nR')\geq (n+1)^2-n^2. Thus we have that dim(R')=\infty(cf. Eisenbud’s ‘Commutative algebra with a view towards algebraic geometry’, Exercise 9.6).

Yet the good news for algebraic geometers is that for noetherian local rings, their dimensions are always finite. We can find this result in Atiyah-Macdonald’s ‘commutative algebra’, the last chapter on dimension theory. The usefulness of the dimensions of a ring gives a good criterion for a variety to be non-singular at a point. For that, we need the concept of regular rings. Let R be a noetherian local ring with maximal ideal \mathfrak{m} and residue field k=R/\mathfrak{m}, then we say that R is regular if dim_k(\mathfrak{m}/\mathfrak{m}^2)=dim(R).

Let Y\subset\mathbb{A}^n_k be an affine variety and P is a point in Y, then Y is non-singular at P if and only if \mathfrak{O}_{Y,P} is regular.

The importance of this result is that it gives an intrisinc description of singular points of a variety. Of course, this criterion can be generalized to other situations since it is a local criterion.

Now let’s return to our origin: we say that a scheme X is regular of co-dimension one if all its local rings \mathfrak{O}_{X,x} of dimension 1 are regular rings. So it is clear that nonsingular varieties are regular of co-dimension 1(considering its associated scheme). Another important class of examples is noetherian normal schemes. Recall that a normal scheme X is a scheme such that all of its local rings \mathfrak{O}_{X,x} are integrally closed. We can use Proposition 9.2 of Atiyah-Macdonald’s ‘commutative algebra’ to show that if a noetherian local ring of dimension one is integrally closed, then it is regular. Note that the word ‘co-dimension’ refers to the dimension of the local rings.

In this post we shall assume that a scheme X is noetherian integral separated, regular of co-dimension one. We will see why we make these assumptions on X.

A prime divisor of X is a closed integral subscheme Y of X of co-dimension 1(here integral scheme correspond to algebraic varieties, ‘integral’ means reduced and irreducible). Then a Weil divisor of X is define to be an element of the free abelian group on the basis of all prime divisors of X. If D=\sum_in_iY_i is a Weil divisor on X, we say that D is effective if n_i\geq 0 for all i.

Note that if Y is a prime divisor of X, then by definition, Y is irreducible, thus it has a (unique) generic point, say, y_0\in Y. We have that \mathfrak{O}_{Y,y_0} is a discrete valuation ring(this also comes from Atiyah-Macdonald’s book, prop 9.2) with quotient field K, and we write v_Y for this valuation.

We can show that under the assumptions, K is equal to the function field on X. Now for any f\in K^*, which is also an element in the quotient field of \mathfrak{O}_{X,y_0} where x is the generic point of a prime divisor Y, we set (f)=\sum_Yv_Y(f)Y, which is thus a Weil divisor on X(if we admit the result that v_Y(f)=0 for almost all such Y). We call (f) a principal Weil divisor on X, and we say that two Weil divisors are linearly equivalent if their difference is a principal Weil divisor.

We will say something more on the properties of Weil divisors later on. For the present, we will turn to the definition of Cartier divisors.

Pierre Emil Jean Cartier

Pierre Emil Jean Cartier(photo from here)

Cartier divisors

Suppose again that X is a scheme and U=Spec(A)\subset X is an affine open subset of X. We write S to be the subset of A consisting of non-zero divisors and K(U)=S^{-1}A, which we call the total quotient ring of A(or the total fraction ring of A). For general open subset U\subset X, we set S to be the subset of \Gamma(U,\mathfrak{O}_X) consisting of elements f\in \Gamma(U,\mathfrak{O}_X) such that f_x is not a zero-divisor for each x\in U. Then we define a presheaf of rings on X to be: for each U, the ring is S^{-1}\Gamma(U,\mathfrak{O}_X). Then we call the associated sheaf \mathfrak{K} to be the sheaf of total quotient rings of \mathfrak{O}_X. And \mathfrak{K}^* is the sheaf of multiplicative groups, which consists of invertible elements in the sheaf \mathfrak{K}. Similarly, we have \mathfrak{O}_X^*.

Now we define a Cartier divisor to be a global section of the sheaf \mathfrak{K}^*/\mathfrak{O}_X^* the sheaf of multiplicaitve groups(since these groups are abelian, we shall write them additively if there is no ambiguity). The set of Cartier divisors on X clearly forms an abelian group. A Cartier divisor is principal if it is in the image of the map \Gamma(X,\mathfrak{K}^*)\rightarrow\Gamma(X,\mathfrak{K}^*/\mathfrak{O}_X^*). Two Cartier divisors are called linearly equivalent if they differ by a principal Cartier divisor.

Weil divisors v.s. Cartier divisors

At first sight, there is no relation between Weil divisors and Cartier divisors. Yet the following proposition tells us that in special cases these two are in fact the same thing.

Suppose X is an integral, separated noetherian scheme. Suppose also that all its local rings are UFDs, then the abelian group Div(X) of Weil divisors on X and the group of Cartier divisors \Gamma(X,\mathfrak{K}^*/\mathfrak{O}_X^*) are isomorphic. Besides, the principal Weil divisors corresponds to the principal Cartier divisors.

We shall prove this proposition in the next post.

notes on topological K-theory

This post is about topological K-theory. As for general information on this branch of mathematics, let’s quote some phrases from wikipedia: K-theory, roughly speaking, is the study of certain invariants of large matrices. In this post we will touch a very little bit of this K-theory. The materials of this post are taken from Husemoller’s book, ‘Fibre Bundles'(GTM82) and Hatcher’s bool, ‘algebraic topology’.

Preliminaries

Here we list some foundational concepts for understanding K-theory.

Homotopy is the most basic one among these concepts. For any two topological spaces X and Y and any two continuous maps f,g:X\rightarrow Y, we say that f is homotopic to g is there is a continuous map F:X\times[0,1]\rightarrow Y such that F(x, 0)=f(x),F(x,1)=g(x),\forall x\in X, here the unit interval [0,1] is given the Euclidean topology. In other words, f is homotopic to g if f can vary continuously to g. It is not difficult to verify that homotopy is an equivalence relation. One major concern of algebraic topology is to classify topological spaces up to homotopy. The category of topological spaces with maps up to homotopy equivalence is a quotient category of the category of topological spaces with continuous maps.

Sometimes we study also based spaces and maps. A based space is a topological space with a point in it, (X,x_0). A map between two based spaces (X,x_0),(Y,y_0) is a continuous map f,X\rightarrow Y such that f(x_0)=y_0. The category of based topological spaces with maps up to homotopy equivalence is a quotient category of the topological spaces with continuous maps.

Next let’s define homotopy groups. A loop in a based topological space (X,x_0) is a continuous map f:[0,1]\rightarrow (X,x_0) such that f(0)=f(1)=x_0. Up to homotopy, a loop has an inverse, two loops can compose, and the constant map makes the set of loops in (X,x_0) into a group, which we write as \pi_1(X,x_0). We write \Omega(X) for the space of loop spaces. This is a subspace of the map space Map(I,X), to which we give the compact open topology. This is again a based space with base point the constant map from I to (X,x_0).

In order to introduce higher order homotopy groups, we need some operations in the category of (based) spaces. For two based spaces (X,x_0),(Y,y_0), we define their based union to be X\vee Y to be X\sqcup Y/(x_0\sim y_0)(with base point x_0=y_0), the is wedge product to be X\wedge Y=X\times Y/((x,y_0)\sim(x_0,y_0)\sim(x_0,y)) with base point (x_0,y_0). These two operations are really the analog of the disjoint union and Cartesian product in the category of topological spaces. We write C(X) for the wedge product of (X,x_0) and (I,0) where I=[0,1] and S(X) the wedge product of (X,x_0) and (S^1,0) where S^1=I/(0\sim1). C(X) is called the cone of X, and S(X) is called the suspension of X. It is remarkable to see that S^{n+1}=S(S^n), that is, the n+1-sphere is the wedge product of an n-sphere. It is not hard to imagine this identification, yet it takes some effort to write it down.

Next we say that a multiplication on a based space (Y,y_0) is a map of based spaces f:Y\times Y\rightarrow Y. Note that this f induces in an obvious way a map f_X: Map_0(X,Y)\times Map_0(X,Y)\rightarrow Map_0(X,Y) where Map_0(X,Y) is the spaces of maps of based spaces (X,x_0),(Y,y_0) given the subspace topology induced from Map(X,Y), the space of continuous maps from X to Y, which is given the compact open topology. A co-multiplication on (X,x_0) is a map g:X\rightarrow X\vee X. Again this g induces a map g^X:Map_0(X,Y)\times Map_0(X,Y)\rightarrow Map_0(X,Y)(indeed, for any two i,j:(X,x_0)\rightarrow(Y,y_0), we can define g^Y(i,j)\in Map_0(X,Y) as follows: for any x\in X, if g takes x to the first copy of X in X\vee X, then we set g^Y(i,j)(x)=i(x), otherwise g^Y(i,j)(x)=j(x). The importance of based spaces’ assumption is that, at the point x_0, g^Y(i,j) is well defined).

Are there some examples of multiplications and co-multiplications? In fact the loop space \Omega(Y) is a multiplication space, note that for any \alpha,\beta\in \Omega(Y), the natural composition of \alpha,\beta gives the multiplication f: \Omega(Y)\times \Omega(Y)\rightarrow \Omega(Y), (\alpha,\beta)\mapsto \alpha\circ\beta. The suspension S(X) is a co-multiplicaiton space, g:S(X)\rightarrow S(X)\vee S(X) takes (x,t)\in S(X)=X\wedge S^1 to ((x,2t),*) if 0\leq t\leq 1/2 and to (*,(x,2t-1)) otherwise, where * is the base point of S(X).

For any (X,x_0) with a co-multiplication or (Y,y_0) with multiplication, we see easily that Map_0(X,Y) has a monoid structure, yet in general this is not a group. We call (X,x_0) a co-H-space if Map_0(X,Y) is a group for any based space (Y,y_0), and we call (Y,y_0) an H-space if Map_0(X,Y) is a group for any (X,x_0). We can verify that S(X) is an H-space and \Omega(Y) is a co-H-space. At first sight, this is quite magical, at least this is so for me. Yet let’s recall how to show that S^1 is an H-space. To construct the inverse of a loop in (X,x_0), we use essential the identification S^1=I/\partial I. In general, this is also the case. Recall that S(X)=X\wedge S^1, a quotient space of X\times I, for any \alpha\in Map_0(S(X),Y), we define \alpha'(x,t)=\alpha(x,1-t)\in Y. Now use the co-multiplication structure g:S(X)\rightarrow S(X)\vee S(X), and we write g^Y(\alpha,\alpha')=\beta, then \beta(x,t)=\alpha(x,2t) if 0\leq t\leq 1/2, and \beta(x,t)=\alpha'(x,2t-1)=\alpha(x,2-2t) otherwise. We need to show that \beta is homotopic to the constant map const: S(X)\rightarrow Y,(x,t)\mapsto y_0. This is not hard to verify, just as in the case of S^1. We omit the case that \Omega(Y) is an H-space.

Now finally we can define the homotopy group. The n-th homotopy group(n>0) of a based space (X,x_0) is \pi_n(X,x_0)=Map_0(S^n,X).

One surprising result is that \pi_n(X,x_0) is abelian for all n>1. The usefulness of homotopy groups comes from Whitehead’s theorem:

For two CW-complexes X, Y, if the map f:X\rightarrow Y induces isomorphisms f_*: \pi_n(X)\rightarrow \pi_n(Y) for all n, then f is a homotopy equivalence.

So this theorem justifies the importance of the homotopy groups, yet they are very hard to compute.

In the next post we shall say something about fibre bundles, which turns out to be a fundamental tool in the development of K-theory.

Notes on quasi-coherent sheaves and coherent sheaves-1

Sheaves are great generalizations of function spaces on a topological space. Yet not all sheaves have nice properties, especially when it comes to computing their topological invariants, the homological groups and cohomological groups. So we should try to find some nicely-behaved sheaves. It was perhaps due to J-P.Serre in his article ‘Faiseaux Algèbriques Cohérents’ that we found at last some class of sheaves which indeed have nice properties, it is the quasi-coherent sheaves and coherent sheaves that we are going to talk about in this post. In this post, we will give the definitions and basic properties of these sheaves. The references for this post are Hartshorne’s Algebraic Geometry and Qing Liu’s Algebraic Geometry and Arithmetic Curves as well as some posts from the Internet.

Sheaf Modules

Given a topological space X and a sheaf \mathfrak{O}_X of (locally) ringed space, just as in the case of given a ring, where we would like to consider all the modules over this ring, here we also would like to consider the category of \mathfrak{O}_X-modules. In other words, an \mathfrak{O}_X-module is a sheaf \mathfrak{F} over X such that for any open subset U of X, \mathfrak{F}(U) is a module over \mathfrak{O}_X(U). We define morphisms between \mathfrak{O}_X-modules in an obvious way, thus we get a category Mod(\mathfrak{O}_X), and when there is no ambiguity, we write simply Mod(X).

It is not difficult to verify that Mod(X) is an abelian category. So there is a natural question arising: does this category admit enough projective or injective objects? Very interestingly, Grothendieck showed in his famous Tohoku paper ‘Sur quelques points d’algèbre homologique’ that Mod(X) indeed has enough injectives. Nowadays the proof of this result becomes fair standard, and let’s copy such one from Hartshorne’s book:

For each x\in X, we consider the stalk \mathfrak{F}_x, which has a morphism \mathfrak{F}_x\rightarrow I_x where I_x is an injective module over \mathfrak{O}_{X,x}(interestingly, the existence of enough injectives in a category of modules over a ring is more difficult to prove than that of enough projectives). Now that there is a canonical injection j_x:\{x\}\rightarrow X for each point x\in X, thus if we view I_x also as the sheaf on the single point space \{x\}, then the push-forward map gives that j_{x,*}(I_x) is a sheaf on X. Now we take the products \mathfrak{I}=\prod_{x\in X}j_{x,*}(I_x), which is thus a sheaf over X. Next we show that \mathfrak{F} maps injectively into \mathfrak{I} and \mathfrak{I} is injective. Let’s first show that second point. Now that \mathfrak{I} is a product of these sheaves I_x, thus for any sheaf \mathfrak{G} over X, we have that Hom_{\mathfrak{O_X}}(\mathfrak{G},\mathfrak{I})=\prod_{x\in X}Hom_{\mathfrak{O}_X}(\mathfrak{G},j_{x,*}(I_x)). Yet one has that Hom_{\mathfrak{O}_X}(\mathfrak{G},j_{x,*}(I_x))=Hom_{\mathfrak{O}_{X,x}}(\mathfrak{G}_x,I_x) since I_x=\mathfrak{I}_x. Now we can in passing show the first point: take \mathfrak{G}=\mathfrak{F}, since for each x\in X, there is an injective map \mathfrak{F}_x\rightarrow\mathfrak{I}_x by construction, thus there is a morphism \mathfrak{F}\rightarrow\mathfrak{I}, which is thus also injective. Note that Hom_{\mathfrak{O_X}}(\mathfrak{G},\mathfrak{I})=\prod_{x\in X}Hom_{\mathfrak{O}_X}(\mathfrak{G},j_{x,*}(I_x)), the functor of taking stalks for all x\in X, \mathfrak{F}\mapsto\mathfrak{F}_x is exact(we emphasize on the word ‘all’), and at each stalk, the functor \mathfrak{G}_x\mapsto Hom_{\mathfrak{O}_{X,x}}(\mathfrak{G}_x,I_x) is also exact since I_x is injective, thus the functor Hom_{\mathfrak{O}_X}(\cdot, \mathfrak{I}) is exact, thus \mathfrak{I} is injective.

So we have that any Mod(X) has enough injectives, yet the problem of enough projectives is more subtle: not every Mod(X) has enough projectives. Perhaps it is better to give some counterexamples(these are taken from the answers in this post). Moreover, it is easy to see that above argument does not apply to the projective case, the essential obstacle is that, Hom_{\mathfrak{O}_X}(\mathfrak{G},j_{x,*}(I_x))=Hom_{\mathfrak{O}_{X,x}}(\mathfrak{G}_x,I_x). Note that j_x^{-1} is left adjoint to the functor j_{x,*}, which gives the above identity. So the existence of enough injectives comes essentially from the point that the stalk functor at one point is left adjoint to the extension by zero functor. And that is why we can use this argument to show the existence of enough projectives.

Now there is a natural functor, the global section functor, \Gamma(X,\cdot):Mod(X)\rightarrow Mod(\mathfrak{O}_X(X)), from Mod(X) to the category of \mathfrak{O}_X(X)-modules.

Unfortunately, this functor is not an exact functor(in some sense, this is also formate for us, since otherwise there would not be such a rich theory of sheaf cohomology). Suppose that X=\mathbb{A}^1_k the affine line over a field k, and P,Q\in X two distinct closed points and U=X-\{P,Q\}, the open complement of them in X. Now we take the constant sheaf \mathfrak{O}_X=\mathbb{Z}_X over X. Now we set \mathbb{Z}_U=i_!(\mathbb{Z}_X|_U) where i:U\rightarrow X is the inclusion map and \mathbb{Z}_Y=j_*(\mathbb{Z}_X|_Y) where j:Y=\{P,Q\}\rightarrow X is also the inclusion. We have an exact sequence 0\rightarrow \mathbb{Z}_U\rightarrow\mathbb{Z}_X\rightarrow\mathbb{Z}_Y\rightarrow0. Yet consider what is \mathbb{Z}_Y now: by definition, \mathbb{Z}_Y is a sheaf on X such that for each open subset V\subset X, there is \mathbb{Z}_Y(V)=\mathbb{Z}_X|_Y(j^{-1}(V)). If P\in V,Q\not\in V, we have that \mathbb{Z}_Y(V)=\mathbb{Z}, similar for the case P\not\in V,Q\in V. If P,Q\in V, then \mathbb{Z}_Y(V)=\mathbb{Z}^2. Taking global section, we see that \mathbb{Z}_Y(X)=\mathbb{Z}^2, \mathbb{Z}_X(X)=\mathbb{Z}, \mathbb{Z}_U(X)=0, thus the global section sequence 0\rightarrow 0\rightarrow \mathbb{Z}\rightarrow\mathbb{Z}^2\rightarrow0 is no longer exact.

So now we may wonder what conditions we should pose on the underlying topological space or on the sheaves to ensure that H^i(X,\mathfrak{F})=0 for i>0, where we write H^i(X,\mathfrak{F}) for the i-th right derived functor of the global section functor.

Quasi-coherent sheaves

Quasi-coherent sheaves partially answers this question. Now let’s give the definition:

Suppose \mathfrak{F} is an \mathfrak{O}_X-module, we say that \mathfrak{F} is generated by its global sections at x\in X if the canonical morphism \mathfrak{F}(X)\bigotimes_{\mathfrak{O}_X(X)}\mathfrak{O}_{X,x}\rightarrow \mathfrak{F}_x is surjective. We say that \mathfrak{F} is generated by its global sections if it is so at each point x\in X.

It is not hard to see that \mathfrak{F} is generated by its global sections if and only if there is an index set I and a surjective morphism \mathfrak{O}_X^{\bigoplus |I|}=\mathfrak{O}_X^{(I)}\rightarrow\mathfrak{F}\rightarrow0.

We say that \mathfrak{F} is quasi-coherent if for each x\in X, there is an open neighborhood x\in U\subset X such that there is an exact sequence \mathfrak{O}_X^{(J)}|_U\rightarrow\mathfrak{O}_X^{(I)}|_U\rightarrow \mathfrak{F}|_U\rightarrow0.

So this means that a quasi-coherent sheaf \mathfrak{F} locally admits free resolutions(of exact sequence of length 2).

Quasi-coherent sheaves behave quite nicely over an affine scheme. This can be seen from the result below.

Let’s fix a commutative ring R and set X=Spec(R). Now if M is a module over R, then we can give a natural \mathfrak{O}_X-module M^{\sim} constructed as follows: for each affine open subset D(r) of X with r\in R, we set M^{\sim}(D(r))=M_r=M\bigotimes_RR_r. For general open subsets, we take the direct limit. It is easy to see that M^{\sim}(X)=M, and (M^{\sim})_{\mathfrak{p}}=M_{\mathfrak{p}} for any \mathfrak{p}\in X. Clearly, we have that (\bigoplus_iM_i)^{\sim}\simeq \bigoplus_iM^{\sim}_i where each M_i is an R-module. One surprising result is that

A sequence of R-modules L\rightarrow M\rightarrow N is exact if and only if L^{\sim}\rightarrow M^{\sim}\rightarrow N^{\sim} is so. This is really an important proposition, and let’s give a proof of it.

Suppose that L\rightarrow M\rightarrow N is exact, then we know that R_{\mathfrak{p}} is flat, thus the sequence L\bigotimes_RR_{\mathfrak{p}}\rightarrow M\bigotimes_RR_{\mathfrak{p}}\rightarrow N\bigotimes_RR_{\mathfrak{p}} is again exact, yet the latter is just (L^{\sim})_{\mathfrak{p}}\rightarrow(M^{\sim})_{\mathfrak{p}}\rightarrow(N^{\sim})_{\mathfrak{p}}, which shows that L^{\sim}\rightarrow M^{\sim}\rightarrow N^{\sim} is indeed exact. Conversely, suppose that L^{\sim}\rightarrow M^{\sim}\rightarrow N^{\sim} is exact, then at stalk level, we have that (L^{\sim})_{\mathfrak{p}}\rightarrow(M^{\sim})_{\mathfrak{p}}\rightarrow(N^{\sim})_{\mathfrak{p}} is also exact, so is L\bigotimes_RR_{\mathfrak{p}}\rightarrow M\bigotimes_RR_{\mathfrak{p}}\rightarrow N\bigotimes_RR_{\mathfrak{p}}. Now consider the original sequence L\xrightarrow{f} M\xrightarrow{g} N where g\circ f=0 since the sheaf sequence is exact. We have that (Ker(g)/Im(f))_{\mathfrak{p}}=0 for all \mathfrak{p}\in Spec(R). This shows that Ker(g)/Im(f)=0, thus L\rightarrow M\rightarrow N is indeed exact.

So in the case of affine schemes, for this type of sheaves, we see that the global section functor is exact.

Then what is the relation of this kind of sheaves with quasi-coherent shaves that we have just introduced? Well, note that if M is an R-module, then it admits a free presentation, R^{(J)}\rightarrow R^{(I)}\rightarrow M\rightarrow 0 is an exact sequence. And therefore (R^{(J)})_r\rightarrow (R^{(I)})_r\rightarrow M_r\rightarrow 0 is again exact since R_r is flat module over R for each r\in R. So, this means that for each \mathfrak{p}\in X, there is an open neighborhood \mathfrak{p}\in D(r)\subset X such that (R^{(J)})^{\sim}|_{D(r)}\rightarrow (R^{(I)})^{\sim}|_{D(r)}\rightarrow M|_{D(r)}\rightarrow 0 is exact since M|_{D(r)}=M_r, similar for the other two.

In fact, we can show that quasi-coherent sheaves locally look like this, that is:

Proposition: let X be a scheme(not necessarily affine), and \mathfrak{F} be an \mathfrak{O}_X-module. Then \mathfrak{F} is quasi-coherent if and only for each affine open subset U\subset X, there is \mathfrak{F}(U)^{\sim}\simeq \mathfrak{F}|_U.

This is really exciting news: quasi-coherent sheaves locally look all like sheaf modules constructed as above, which, as we have seen, have very nice properties.

Let’s also try to give a proof of this result. For this, we need a lemma:

Lemma: let X be a scheme which is Noetherian or separated and quasi-compact, \mathfrak{F} be a quasi-coherent sheaf over X, then for each f\in \mathfrak{O}_X(X), the canonical morphism

\mathfrak{F}(X)_f=\mathfrak{F}(X)\bigotimes_{\mathfrak{O}_X(X)}\mathfrak{O}_X(X)_f\rightarrow \mathfrak{F}(X_f)

is an isomorphism, where X_f=\{x\in X|f_x\in \mathfrak{O}_{X,x}^*\}.

Note that since \mathfrak{F} is quasi-coherent, for each x\in X, there is an affine open subset x\in U\subset X such that there is a free representation \mathfrak{O}_X^{(J)}|_U\rightarrow \mathfrak{O}_X^{(I)}|_U\xrightarrow{f} \mathfrak{F}|_U\rightarrow 0. Taking global sections, we set M=f(U), then we have that \mathfrak{O}_X^{(J)}|_U\rightarrow \mathfrak{O}_X^{(I)}|_U\xrightarrow{f} M^{\sim}\rightarrow 0 is exact, which implies that M^{\sim}\simeq \mathfrak{F}|_U, thus we get that M\simeq \mathfrak{F}(U). In other words, for each x\in X, there is an affine open subset x\in U\subset X such that \mathfrak{F}|_U\simeq \mathfrak{F}(U)^{\sim}. By assumption, X is quasi-compact, thus we can find a finitely many such U_i covering X. Moreover, we set V_i=U_i\bigcap X_f. Let’s spell out what V_i is. Recall that X_f consists of points x\in X such that f_x is invertible in \mathfrak{O}_{X,x}, in other words, if x is viewed as a prime ideal in R_i where U_i=Spec(R_i), then f|_{U_i}\not\in x. So we see that V_i=D(f|_{U_i}). Now for each i, we have maps \mathfrak{F}(U_i)_f=\mathfrak{F}(U)\bigotimes_{\mathfrak{O}_X(U_i)}\mathfrak{O}_X(U_i)_f\rightarrow \mathfrak{F}(V_i)=\mathfrak{F}(D(f|_{U_i})), this is an isomorphism since \mathfrak{F}|_{U_i}\simeq \mathfrak{F}(U_i)^{\sim}. Now using the routine commutative diagram,

pic-my-blog-notes on quasi-coherent sheaves and coherent sheaves

The horizontal sequences are exact according to the definition of scheaves, and the first, the third and the fourth vertical arrows are isomorphisms(we should ensure that U_i\bigcap U_j is again affine, which indeed is the case since X is Noetherian or separated, so we used the hypothesis in an essential way), and thus we get that the second vertical arrow is also an isomorphism, which finishes the proof of this lemma.

Return to our original proposition. Suppose that U is an affine open subset of X, then for each f\in \mathfrak{O}_X(U), we have that \mathfrak{F}(U)_f\simeq \mathfrak{F}(D(f)) by the above lemma, thus \mathfrak{F}(U)^{\sim}\simeq \mathfrak{F}|_U. The converse is clear.

At last, let’s give a proposition which reveals the exactness of the global section functor in a larger extent:

Proposition: let X be an affine scheme, and 0\rightarrow \mathfrak{F}\rightarrow\mathfrak{G}\rightarrow\mathfrak{H}\rightarrow 0 be an exact sequence of sheaves over X. Suppose that \mathfrak{F} is quasi-coherent, then the sequence

0\rightarrow \mathfrak{F}(X)\rightarrow\mathfrak{G}(X)\rightarrow\mathfrak{H}(X)\rightarrow 0

is exact.

We will show this result in the next post, which will use Cech cohomology.

Notes on semisimple rings and modules

In this post I want to say something on semisimple rings and semisimple modules.

Basic settings

Suppose that R is a ring with unit and M is a module over R. We say that M is a simple module if it does not contain other submodule other than 0 and M. Then what is a semisimple module over R? We say that M is semisimple if any submodule M' of M is a direct summand of M. Note that this is a very strong condition on the module. Recall that this kind of phenomena occurs most naturally to vector spaces over a field. So, in this way, we can say that semisimple modules are the modules most similar to vector spaces. Besides, it is easy to see that a simple module is automatically a semisimple module.

Let’s then define what a semisimple ring is. We say that R is a semisimple ring if the R-module R(the ring R acts by left multiplication on the abelian group R) is a semisimple module over R. So here we should really be careful about semisimple rings because we are apt to thinking that R is a semisimple ring if any subring of R is a direct summand of R, which is completely false according to the definition. In fact, R is a semisimple ring if any ideal R is a direct summand of the module R.

Now it is not difficult to see that: R is a semisimple ring if and only if any module M over R is semisimple. The ‘if’ part is clear. For the ‘only if’ part, note that M is a quotient of some R^n, which is again semisimple. Now consider the exact sequence

0\rightarrow Ker(p)\rightarrow R^n\xrightarrow{p}M\rightarrow 0

Note that Ker(p) is a submodule of R^n, thus is a direct summand of R^n, so M is also a direct summand of R^n, thus is also semisimple.

Here we have used a crucial property of semisimple modules over R: if M is semisimple, then its submodules and quotient modules are again semisimple.

So in this way, a semisimple ring R looks very like a field in that all the modules over it are semisimple.

Structure theory of semisimple rings

What is fascinating about the theory of semisimple rings is that there is a nice decomposition of a semisimple ring into some well known algebraic objects. Let’s briefly have a look at this part of the theory.

Suppose that R is a semisimple ring and M is a simple R-module. Now choose any element m\in M, we have a morphism of R-modules

f_m:R\rightarrow M,r\mapsto rm

It is easy to see that Im(f_m) is a submodule of M, and since M is simple, thus either Im(f_m)=M or Im(f_m)=0. If m\neq 0, then 1_Rm=m\in Im(f_m), this shows that Im(f_m)=M for m\neq 0. Thus M is a quotient of R, and since the latter is a semisimple R-module, thus R\simeq M\bigoplus Ker(f_m). Note that this is an isomorphism of R-modules, thus we can view M as an ideal of R. In this way, we find that all simple modules come from ideals of R, so now we shall restrict our attention to simple ideals of R(which is defined in an obvious way to be an ideal of R, which, viewed as an R-module, is a simple module).

Note that if we do not assume that R is commutative, and we consider for the present the left R-modules, then the resulting fact is to consider all the left simple ideals of R.

We write \mathfrak{I} for the set of equivalence classes of left simple ideals of R. For any I\in \mathfrak{I}, we set M_I to be the (direct) sum of the left ideals of R isomorphic to I as R-modules. We next show that M_I is a two-sided ideal of R. For any two representatives I,J\in\mathfrak{I}, using Schur’s lemma, we see that IJ=0 if I\not\simeq J. Thus we have that M_IM_J=0 in this case. So, we see that these M_I are two-sided ideals. Now that R is semisimple, we can write R=\sum_{I\in\mathfrak{I}}M_I. Using the (finite) decomposition of 1_R=\sum_I e_I, we see that R=\sum_IM_I is in fact a finite sum and R=\bigoplus_I M_I. The by-product of this process is the elements e_I for each I\in\mathfrak{I}(which is now a finite set). These elements have very nice properties(suppose now I,J\in\mathfrak{I} are isomorphism classes, not representatives of these classes):

e_Ie_J=0,I\neq J; e_Ie_I=e_I;\sum_I e_I=1_R

Note that these properties give us another description of the above decomposition R=\bigoplus_I Re_I. This is an isomorphism of R-modules. Yet we have something more: each Re_I is itself a ring. Let’s examine this nice result. Note that we have 1_R=\sum_I e_I, thus for any r\in R, r=1_Rr=\sum_Ie_Ir=e1_R=\sum_Ire_I. Since this is a direct sum, thus according to the uniqueness of the decomposition, we get that e_Ir=re_I for all I\in\mathfrak{I}. Now for any re_I,r'e_I\in Re_I, we have that (re_I)(r'e_I)=r(e_Ir')e_I=r(r'e_I)e_I=rr'e_I, which, combined with some other simple verifications, shows that Re_I is indeed a ring. So, in this sense,

R\simeq \bigoplus_I Re_I

is also an isomorphism of rings(all of these use essentially the fact that R is semisimple).

Simple rings and their structures

So to study the properties of R, it suffices to study these M_I. These M_I are in fact simple rings in the following sense: a ring A is said to be simple if A is semisimple and it contains exactly one isomorphism class of left simple ideals. We will write a simple ring as M_I, just to emphasize that fact that the isomorphism class of left simple ideals is I.

Next we will start from I to construct a division ring. This is one of the most interesting part of the theory. We fix a left simple ideal I\neq0 and we set E=End_{M_I}(I). We next show that E is this division ring. Indeed, for any 0\neq f\in E, since I is simple, thus according to Schur’s lemma, f is an isomorphism. Clearly E has a ring structure, so we are done: E is indeed a division ring.

Now let’s give I an E-module structure: for any m\in I,f\in E=End_{M_I}(I), we define f\cdot m=f(m). It is straightforward that this defines an E-module structure on I. Note that E is already a division ring, so as an E-module, I behave very much like a vector space over a field: we have the concept of dimensions. We will come to this point later.

Note until present we only consider one single left simple ideal of M_I. Next we consider the whole ring M_I, which is a direct sum of the left simple ideals, thus is also an E-module. Surprisingly, it has a much nicer description: M_I is isomorphic to the endomorphism ring E'=End_E(I).

Let’s first give a map from one to the other. For any m\in M_I, we set f_m:I\rightarrow I, r\mapsto mr simply the left multiplication, which indeed maps I to I since I is a left ideal. We should verify that f_m\in End_E(I). For any h\in E=End_{M_I}(I), we should show that h\cdot(f_m(r))=f_m(h\cdot r). Indeed, h\cdot(f_m(r))=h(mr), now h is a M_I-linear map, thus h(mr)=m(h(r))=f_m(h(r))=f_m(h\cdot r), so we are done: f_m is indeed an element in End_E(I). Next we have to show that this map

\phi: M_I\rightarrow End_E(I),m\mapsto f_m

is an isomorphism. Note that Ker(\phi) is a two-sided ideal of M_I, thus as the above argument, M_I=Ker(\phi)\bigoplus M_I/Ker(\phi).where both summands have ring structures and contain different isomorphism classes of left simple ideals, thus we must have Ker(\phi)=0 or Ker(\phi)=M_I. If it is the second case, I=1\cdot I=0, which is impossible since I is non-zero, thus Ker(\phi)=0, showing that \phi is injective. It is also easy to show that \phi is surjective, thus we get an isomorphism

M_I\simeq End_E(I)

Note that if E is actually a field(not simply a division ring), then I is a vector space over E. Let’s suppose that dim_E(I)<\infty, then this result shows that the simple ring M_I is the matrix ring End_E(I). This is really a nice description of simple rings.

So, let’s return to the semisimple case: suppose again R is a semisimple ring, then R\simeq \bigoplus_I M_I, each summand is a matrix algebra over some division ring E_I=End_{M_I}(I).

Now we want to concentrate ourselves to some special (semi)simple rings: rings that are algebras over some field.

One special case

Let’s fix a field k. Suppose R is a finite dimensional k-algebra and a semisimple ring, then each M_I is a finite dimensional k-algebra and a simple ring. Now each E_I=End_{M_I}(I) is a division ring and at the same time a k-algebra, which is finite dimensional since End_{M_I}(I)\subset End_k(I).

Now let’s make a simple yet illuminating calculation. Suppose that M_I\simeq I^{\bigoplus d_I}, n_I=dim_{E_I}(I). Then we have that

dim_k(M_I)=d_Idim_k(I)=d_In_Idim_k(E_I)

Yet M_I\simeq End_{E_I}(I), thus

dim_k(M_I)=n_I^2dim_k(E_I)

Combining these two results, we get that

n_Id_I=n_I^2\implies n_I=d_I

So we have that M_I\simeq M_{n_I}(E_I), the matrix ring with coefficients in E_I. And in general, R\simeq\bigoplus_I M_{n_I}(E_I).

If we suppose that k is algebraically closed, we should have that k=E_I for all I.

One simple application

As an application of this result, we consider the representation theory of a finite group G with char(k)\not| |G|. We set R=k[G], the group algebra of dimension dim_k(R)=|G|. We can show that R is semisimple(we will talk about this proof in another post). We supple also that k is algebraically closed, then according to the above result, we have

k[G]\simeq\bigoplus M_{n_I}(k)

In other words, we have |G|=\sum_In_I^2. Besides, each I is an irreducible representation of G, so in the end we get a fundamental result in the representation theory of finite groups

|G|=\sum_Vdim_k(V)^2

where V runs through all the irreducible representations of G.

group cohomology

In this post we will talk about cohomology(from homological algebra), group cohomology and its applications.

Why should we study (co)homology? Of course there are historical motivations. Yet a modern point of view is to compensate for the non-exactness of a functor from a category(with good properties, for example, an abelian category) to another. So, one more fundamental question arises: why should we concern ourselves with exactness of a sequence? Perhaps the motivation for this comes from the fact that a splitting sequence of abelian groups implies that this sequence is exact. This is the most common necessary condition for a sequence to split.

Now let’s get down to some concrete things, we suppose that \mathfrak{C} is an abelian category, one prototype example is the category of modules over a commutative ring with unit. We consider the associated (\mathbb{Z}-)graded category \mathfrak{C}', in which objects are (C_i,\delta_i)_{i\in\mathbb{Z}} where \delta_i:C_i\rightarrow C_{i-1} is a morphism in \mathfrak{C} such that \delta_i\circ \delta_{i-1}=0. So the first motivational question comes: why de we consider this kind of things? One motivation comes from differential manifolds. In the theory of differential manifolds, we have an important operator, the exterior differential operator, d, which acts on differential forms. Differential forms give a \mathbb{Z}(or \mathbb{N})-graded real vector space, and d\circ d=0. This is an important property of d. And the category \mathfrak{C}' can be seen as a generalization of this object.

Now a morphism from (C_i,\delta_i),(D_i,\partial_i) is a sequence of maps f_i:C_i\rightarrow D_i such that \partial_i\circ f_i=f_{i-1}\circ \delta_i. This definition is very reasonable.

For each such object C=(C_i,\delta_i), we consider H_n(C)=Ker(\delta_i)/Im(\delta_{i+1}). This is of course a most fundamental concept in the theory of (co)homology. One straightforward motivation for these H_n(C) is to compensate for the non-exactness of these maps \delta_i. If f:C=(C_i,\delta_i)\rightarrow D=(D_i,\partial_i) is a morphism, then it is easy to verify that f induces maps H_n(f):H_n(C)\rightarrow H_n(D).

Now let’s consider another different question: given two maps f,g:C=(C_i,\delta_i)\rightarrow D=(D_i,\partial_i), when will H(f)=H(g)? This is a rather interesting question. One sufficient condition comes from homotopy theory, which is, at first sight, completely different from homology theory. We say that two such maps f,g are homotopy equivalent if there is a sequence of maps h_n:C_n\rightarrow D_{n+1} such that f_n-g_n=\partial_{n+1}\circ h_n+h_{n-1}\circ \delta_n. It is very easy to show that if f, g are homotopy equivalent, then they induce the same maps H_n(f)=H_n(g),\forall n. This is really a fundamental verification in homology theory. Recall the domains of H_(f),H(g): they are H_n(C)=Ker(\delta_n)/Im(\delta_{n+1}). Thus for any x\in H_n(C), we have that f_n(x)-g_n(x)=\partial_{n+1}\circ h_n(x)+h_{n-1}\circ \delta_n(x). Note that \delta_n(x)=0, besides \partial_{n+1}\circ h_n(x)\in Im(\partial_{n+1}), thus f_n(x)=g_n(x),\forall x\in H_n(C). It is also easy to show that homotopy equivalent is an equivalent relation. Note here why should we need the first term \partial_{n+1}\circ h_n? In fact these two terms can be seen from homotopy theory. That is what we are going to say in the next post.

Clichés transmitted by professors of maths/physics undergraduates in France

episodic thoughts

The latest issue of Gazette des Mathématiciens, published by SMF, is out today.

Many interesting articles there, but I’d like to mention for now the one (pages 53 to 58) by Arnaud Pierrel, a PhD student in Sociology, who writes in the Parité column.

He studies students who are attending Classes Prépa Scientifiques (in short CPGE, that’s 2 years of very intensive undergraduate courses just after high school at the end of which students attempt various competitive exams to enter a variety of Schools, mainly in engineering but also in Math or Computer Science like the École Normale Supérieure in Paris, ÉNS).

One stunning statistic from Pierrel’s article, which underlines the sociological clichés that teachers are (subconsciously?) transmitting to their students,  is the following (I’ve cut some inessential details, translation below):

Dans le cadre de notre recherche sur les CPGE scientifiques, nous avons suivi une promotion…

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