Étale cohomology

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In mathematics, the étale cohomology groups of an algebraic variety or scheme are algebraic analogues of the usual cohomology groups with finite coefficients of a topological space, introduced by Grothendieck in order to prove the Weil conjectures. Étale cohomology theory can be used to construct l-adic cohomology, which is an example of a Weil cohomology theory in algebraic geometry. This has many applications, such as the proof of the Weil conjectures and the construction of representations of finite groups of Lie type.

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[edit] History

Étale cohomology was discovered by Grothendieck (with the help of some suggestions by J.-P. Serre), motivated by the attempt to construct a Weil cohomology theory in order to prove the Weil conjectures. The foundations were worked out by Grothendieck together with Michael Artin in 1963-1964, and published as SGA 4. Grothendieck used étale cohomology to prove some of the Weil conjectures (Dwork had already managed to prove the rationality part of the conjectures in 1960 using p-adic methods), and the remaining conjecture, the analogue of the Riemann hypothesis was proved by Pierre Deligne (1974).

Further contact with classical theory was found in the shape of the Grothendieck version of the Brauer group; this was applied in short order to diophantine geometry, by Yuri Manin. The burden and success of the general theory was certainly both to integrate all this information, and to prove general results such as Poincaré duality and the Lefschetz fixed point theorem in this context.

Grothendieck originally developed etale cohomology in an extremely general setting, working with concepts such as Grothedieck toposes and Grothendieck universes. With hindsight, this machinery proved unnecessary for applications of the étale theory, and Deligne gave a simplified exposition of étale cohomology theory in SGA4½ (Deligne 1977). Grothendieck's original use of Grothendieck universes (whose existence cannot be proved in ZFC) led to some uninformed speculation that etale cohomology and its applications needed axioms beyond ZFC. Etale cohomology can in fact easily be constructed in ZFC (and even in much weaker theories). In practice etale cohomology is used mainly for constuctible sheaves over schemes of fintie type over the integers, and this needs no deep axioms of set theory: it can be even carried out in Peano arithmetic.

Etale cohomology quickly found other applications, for example Deligne and Lusztig used it to construct representations of finite groups of Lie type.

[edit] Motivation

For complex algebraic varieties, invariants from algebraic topology such as the fundamental group and cohomology groups are very useful, and one would like to have analogues of these for varieties over other fields, such as finite fields. (One reason for this is that Weil suggested that the Weil conjectures could be proved using such a cohomology theory.) In the case of cohomology of coherent sheaves, Serre showed that one could get a satisfactory theory just by using the Zariski topology of the algebraic variety, and in the case of complex varieties this gives the same cohomology groups (for coherent sheaves) as the much finer complex topology. However for constant sheaves such as the sheaf of integers this does not work: the cohomology groups defined using the Zariski topology are badly behaved. For example, all complete algebraic curves are homeomorphic for the Zariski topology, so all have the same cohomology groups with integer coefficients, which is certainly not what one wants.

The reason why the Zariski topology does not work well is that it is too coarse: it has too few open sets. There seems to be no good way to fix this by using a finer topology on a general algebraic variety. Grothendieck's key insight was to realize that there is no reason why the more general open sets needed should be subsets of the algebraic variety: the definition of a sheaf works perfectly well for any category, not just the category of open subsets of a space. He defined étale cohomology by replacing the category of open subsets of a space by the category of étale mapping to a space: roughly speaking, these can be thought of as open subsets of finite unbranched covers of the space. These turn out (after a lot of work) to give just enough extra open sets that one can get reasonable cohomology groups for some constant coefficients, in particular for coefficients Z/nZ when n is coprime to the characteristic of the field one is working over.

Some basic intuitions of the theory are these:

  • The étale requirement is the condition that would allow one to apply the implicit function theorem if it were true in algebraic geometry (but it isn't — implicit algebraic functions are called algebroid in older literature).
  • There are certain basic cases, of dimension 0 and 1, and for an abelian variety, where the answers with constant sheaves of coefficients can be predicted (via Galois cohomology and Tate modules).

[edit] Definitions

For any scheme X the category Et(X) is the category of all etale morphisms from a scheme to X. It is an analogue of the category of open subsets of a topological space, and its objects can be thought of informally as "etale open subsets" of X. The intersection of two open sets of a topological space corresponds to the pullback of two etale maps to X.

A presheaf on a topological space X is a contravariant functor form the category of open subsets to sets. By analogy we define an etale preseaf on a scheme X to be a contravariant functor from Et(X) to sets.

A presheaf F on a topological space is called a sheaf if it satisfies the sheaf condition: whenever an open subset is covered by open subsets Ui, and we are given elements of F(Ui) for all i whose restrictions to Ui∩Uj agree for all i, j, then they are images of a unique element of F(U). By analogy, an etale presheaf is called a sheaf if it satisfies the same condition (with intersections of open sets replaced by pullbacks of etale morphisms, and where a set of etale maps to U is said to cover U if the topological space underlying U is the union of their images). More generally, one can define a sheaf for any Grothendieck topology on a category in a similar way.

The category of sheaves of abelian groups over a scheme has enough injective objects, so one can define right derived functors of left exact functors. The étale cohomology groups Hi(F) of the sheaf F of abelian groups are defined as the derived functors of the functor of sections,

F \to \Gamma(F) , \,\!

(where the space of sections Γ(F) of F is F(X)). The sections of a sheaf can be thought of as Hom(Z,F) where Z is the sheaf that returns the integers as an abelian group. The idea of derived functor here is that the sheaf of sections doesn't respect exact sequences as it is not right exact; according to general principles of homological algebra there will be a sequence of functors Hi for i = 0,1, ... that represent the 'compensations' that must be made in order to restore some measure of exactness (long exact sequences arising from short ones). The H0 functor coincides with the section functor Γ.

More generally, if f is a morphism of schemes from X to Y, it induces a map f* from etale sheaves over X to etale sheaves over Y, and its right derived functors are denoted by Rqf*, for q a non-negative integer. In the special case when Y is the spectrum of an algebraically closed field (a point), Rqf*(F) is the same as Hq(F).

Suppose that X is a Noetherian scheme. An abelian étale sheaf F over X is called finite locally constant if it is represented by an étale cover of X. It is called constructible if X can be covered by a finite family of subschemes on each of which the restriction of F is finite locally constant. It is called torsion if F(U) is a torsion group for all etale covers U of X. Finite locally constant sheaves are constructible, and constructible sheaves are torsion. Every torsion sheaf is a filtered inductive limit of constructible sheaves.

[edit] ℓ-adic cohomology groups

In applications to algebraic geometry over a finite field Fq, the main objective was to find a replacement for the singular cohomology groups with integer (or rational) coefficients, which are not available in the same way as for geometry of an algebraic variety over the complex number field. Étale cohomology works fine for coefficients Z/nZ for n coprime to the characteristic, but gives unsatisfactory results for non-torsion coefficients. To get cohomology groups without torsion from etale cohomology one has to take an inverse limit of etale cohomology groups with certain torsion coefficients; this is called ℓ-adic cohomology. Here ℓ stands for any prime number different from p, where p is the characteristic of Fq. One considers, for schemes V, the cohomology groups

H^i(V, \mathbb{Z}/\ell^k\mathbb{Z})

and defines the ℓ-adic cohomology group

H^i(V,\mathbb{Z}_\ell) = \lim_{\leftarrow}H^i(V, \mathbb{Z}/\ell^k\mathbb{Z})

as their inverse limit. Here Zℓ denotes the ℓ-adic integers, but the definition is by means of the system of 'constant' sheaves with the finite coefficients Z/ℓkZ. (There is a notorious trap here: cohomology does not commute with taking inverse limits, and the ℓ-adic cohomology group, defined as an inverse limit, is not the cohomology with coefficients in the etale sheaf Zℓ; the latter cohomology group exists but gives the "wrong" cohomology groups.)

More generally, if F is an inverse system of etale sheaves Fi, then the cohomology of F is defined to be the inverse limit of the cohomology of the sheaves Fi

H^q(X,F) = \lim_\leftarrow H^q(X, F_i)

and though there is a natural map

H^q(X,\lim_\leftarrow F_i) \mapsto \lim_\leftarrow H^q(X, F_i)

this is not usually an isomorphism. An l-adic sheaf is a special sort of inverse system of etale sheaves Fi, where i runs through positive integers, and Fi is a module over Z/liZ and the map from Fi+1 to Fi is just reduction mod Z/liZ.

In the case that V is a non-singular algebraic curve and i = 1, H1 is a free Zℓ-module of rank 2g, dual to the Tate module of the Jacobian variety of V, where g is the genus of V. Since the first Betti number of a Riemann surface of genus g is 2g, this is isomorphic to the usual singular cohomology with Zℓ coefficients for complex algebraic curves. It also shows one reason why the condition ℓ ≠ p is required: when ℓ = p the rank of the Tate module is at most g.

To remove any torsion subgroup from the ℓ-adic cohomology groups and get cohomology groups that are vector spaces over fields of characteristic 0 one defines

H^i(V,\mathbb{Q}_\ell)=H^i(V,\mathbb{Z}_\ell)\otimes\mathbb{Q}_\ell

(though this notation is misleading: Qℓ is neither an etale sheaf nor an ℓ-adic sheaf).

Torsion can occur, and was applied by Mike Artin and David Mumford to geometric questions.

[edit] Properties

In general the ℓ-adic cohomology groups of a variety tend to have similar properties to the singular cohomology groups of complex varieties, except that they are modules over the ℓ-adic integers (or numbers) rather than the integers (or rationals). They satisfy a form of Poincaré duality on non-singular projective varieties, and the ℓ-adic cohomology groups of a "reduction mod p" of a complex variety tend to have the same rank as the singular cohomology groups. A Künneth formula also holds.

For example, the first cohomology group of a complex elliptic curve is a free module of rank 2 over the integers, while the first ℓ-adic cohomology group of an elliptic curve over a finite field is a free module of rank 2 over the ℓ-adic integers, provided l is not the characteristic of the field concerned, and is dual to its Tate module.

There is one way in which ℓ-adic cohomology groups are better than singular cohomology groups: they tend to be acted on by Galois groups. For example, if a complex variety is defined over the rational numbers, its ℓ-adic cohomology groups are acted on by the absolute Galois group of the rational numbers: they afford Galois representations.

Elements of the Galois group of the rationals, other than the identity and complex conjugation, do not usually act continuously on a complex variety defined over the rationals, so do not act on the singular cohomology groups. This phenomenon of Galois representations is related to the fact that the fundamental group of a topological space acts on the singular cohomology groups, because Grothendieck showed that the Galois group can be regarded as a sort of fundamental group. (See also Grothendieck's Galois theory.)

[edit] Calculation of étale cohomology groups

The main initial step in calculating étale cohomology groups of a variety is to calculate them for complete connected smooth algebraic curves X over algebraically closed fields k. The étale cohomology groups of arbitrary varieties can be reduced to this case using analogues of the usual machinery of algebraic topology, such as the spectral sequence of a fibration. For curves the calculation takes several steps, as follows. (The sheaf Gm is the sheaf of non-vanishing functions.)

[edit] Calculation of H1(X, Gm)

The exact sequence of etale sheaves

1\rightarrow G_m\rightarrow j_*G_{m,K}\rightarrow\oplus_{x\in X}i_{x*}Z\rightarrow 1

gives a long exact sequence of cohomology groups

0\rightarrow H^0(G_m)\rightarrow H^0(j_*G_{m,K})\rightarrow\oplus_{x\in X}H^0(i_{x*}Z)\rightarrow H^1(G_m)\rightarrow H^1(j_*G_{m,K})\rightarrow\cdots

Here j is the injection of the generic point, ix* is the injection of a closed point x, and Zx is a copy of Z for each closed point of X. The groups Hi(ix*Z) vanish if i>0 (because ix*Z is a "skyscraper sheaf") and if i=0 they are Z so their sum is just the divisor group of X. Moreover the first cohomology group H1(X, j*Gm,K) is isomorphic to the Galois cohomology group H1(K, K*) which vanishes by Hilbert's theorem 90. Therefore the long exact sequence of etale cohomology groups gives an exact sequence

K\longrightarrow Div(X)\longrightarrow H^1(G_m)\longrightarrow 1

where Div(X) is the group of divisors of X and K is its function field. In particular H1(X, Gm) is the Picard group Pic(X) (and the first cohomology groups of Gm are the same for the étale and Zariski topologies). This step works for varieties X of any dimension (with points replaced by codimension 1 subvarieties), not just curves.

[edit] Calculation of Hi(X, Gm)

The same long exact sequence above shows that if i≥2 then the cohomology group Hi(X, Gm) is isomorphic to Hi(X, j*Gm,K), which is isomorphic to the Galois cohomology group Hi(K, K*). Tsen's theorem implies that that the Brauer group of a function field K in one variable over an algebraically closed field vanishes. This in turn implies that all the Galois groups Hi(K, K*) vanish for i≥1, so all the cohomology groups Hi(X, Gm) vanish if i≥2.

[edit] Calculation of Hi(X, μn)

If μn is the sheaf of nth roots of 1 and n is prime to the characteristic of the field k, then Hi(X,μn) is μn(k) if i=0, the group of n-division points of Pic(X) if n=1, and Z/nZ if n=2, and 0 if n≥3. This follows from the previous results using the long exact sequence

0\rightarrow H^0(X, \mu_n)\rightarrow H^0(X, G_m)\rightarrow H^0(X, G_m)\rightarrow
\rightarrow H^1(X, \mu_n)\rightarrow H^1(X, G_m)\rightarrow H^1(X, G_m)\rightarrow H^2(X, \mu_n)\rightarrow H^2(X, G_m)

of the Kummer exact sequence of etale sheaves

1\rightarrow\mu_n\rightarrow G_m\xrightarrow{n}G_m\rightarrow 1.

and inserting the known values H0(X,Gm) = k*, H1(X,Gm) = Pic(X), and Hi(X,Gm)=0 for i≥2. In particular we get an exact sequence

1\rightarrow H^1(X, \mu_n)\rightarrow Pic(X)\xrightarrow{\times n} Pic(X)\rightarrow H^2(X, \mu_n)\rightarrow 1

If n is divisible by p this argument breaks down because pth roots of 1 behave strangely over fields of characteristic p. In the Zariski topology the Kummer sequence is not exact on the right, as a non-vanishing function does not usually have an nth root locally for the Zariski topology, so this is one place where the use of the etale topology rather than the Zariski topology is essential.

[edit] Calculation of Hi(X, Z/nZ)

By fixing a primitive nth root of 1 we can identify the group Z/nZ with the group μn of nth roots of 1. The étale group Hi(X,Z/nZ) is then a free module over the ring Z/nZ of rank 1 if i=1, 2g if i=1, 1 if i=2, and 0 if i≥ 3 (where g is the genus of the curve X). This follows from the previous result, using the fact that the Picard group of a curve is the points of its Jacobian variety, an abelian variety of dimension g, and if n is coprime to the characteristic then the points of order dividing n in an abelian variety of dimension n over an algebraically closed field form a group isomorphic to (Z/nZ)2g. These values for the étale group Hi(X,Z/nZ) are the same as the corresponding singular cohomology groups when X is a complex curve.

[edit] Examples of étale cohomology groups

  • If X is the spectrum of a field K with absolute Galois group G, then étale sheaves over X correspond to continuous sets (or abelian groups) acted on by the (profinite) group G, and étale cohomology of the sheaf is the same as the group cohomology of G.
  • If X is a complex variety, then étale cohomology with finite coefficients is isomorphic to singular cohomology with finite coefficients. (This does not hold for integer coefficients.) More generally the cohomology with coefficients in any constructible sheaf is the same.
  • If F is a coherent sheaf (or Gm) then the etale cohomology of F is the same as Serre's coherent sheaf cohomology calculated with the Zariski topology (and if X is a complex variety this is the same as the sheaf cohomology calculated with the usual complex topology).
  • For abelian varieties and curves there is an elementary description of l-adic cohomology. For abelain varieties the first l-adic cohomology group is the dual of the Tate module, and the higher cohomology groups are given by its exterior powers. For curves the first cohomology group is the first cohomology group of its Jacobian. This explains why Weil was able to give a more elementary proof of the Weil conjectures in these two cases: in general one expects to find an elementary proof whenever there is an elementary description of the l-adic cohomology.

[edit] Poincaré duality and cohomology with compact support

The étale cohomology groups with compact support of a variety X are defined to be

H_c^q(X,F) = H^q(Y, j_!F)

where j is an open immersion of X into a proper variety Y and j! is the extension by 0 of the étale sheaf F to Y. This is independent of the immersion j. If X has dimension at most n and F is a torsion sheaf then these cohomology groups with comact support vanish if q>2n, and if in addition X is affine of finite type over a separably closed field they vanish for q>n.

More generally if f is a separated morphism of finite type from X to S (with X and S Noetherian) then the higher direct images with compact support Rqf! are defined by

Rqf!(F) = Rqg * (j!F)

for any torsion sheaf F. Here j is any open immersion of X into a scheme Y with a proper morphism g to S (with f=gj), and as before the definition does not depend on the choice of j and Y. Cohomology with compact support is the special case of this with S a point. If f is a separated morphism of finite type then Rqf! takes constructible sheaves on X to constructible sheaves on S. If in addition the fibers of f have dimension at most n then Rqf! vanishes on torsion sheaves for q>2n. If X is a complex variety then Rqf! is the same as the usual higher direct image with compact support (for the complex topology) for torsion sheaves.

If X is a smooth algebraic variety of dimension N and n is coprime to the characteristic then the there is a trace map

Tr: H_c^{2N}(X, \mu_n^N) \mapsto Z/nZ

and the bilinear form Tr(a∪b) with values in Z/nZ identifies each of the groups

H^i_c(X,\mu_n^N)

and

H2N − i(X,Z / nZ)

with the dual of the other. This is the analogue of Poincaré duality for étale cohomology.

[edit] An application to curves

This is how the theory could be applied to the local zeta-function of an algebraic curve.

Theorem. Let X be a curve of genus g defined over the finite field with p elements. Then for every n greater or equal 1 one has

\#X(\mathbb F_{p^n}) = 1 -\sum_{i=1}^{2g} \alpha_i^n+p^n,

where αi are certain algebraic numbers satisfying |\alpha_i|=\sqrt p.

Notes

  • This agrees with the projective line being a curve of genus 0 and having pn+1 points.
  • We see that number of points on any curve is 'rather close' to that of the projective line.

Idea of proof

According to the Lefschetz fixed point theorem, the number of fixed points of any morphism f:X\to X is equal to the sum

\sum_{i=0}^{\dim X}(-1)^i\mathrm{Tr} f|_{H^i(X)}.

This formula is valid for ordinary topological varieties and ordinary topology, but it is wrong for most algebraic topologies. However, this formula does hold for étale cohomology (though this is not so simple to prove).

The points of X that are defined over Fpn are those fixed by Fn, where F is the Frobenius automorphism in characteristic p.

The étale cohomology Betti numbers of X in dimensions 0, 1, 2 are resp. 1, 2g, and 1.

According to all of these,

\#X(\mathbb F_{p^n}) = \mathrm{Tr} F^n|_{H^0(X)} -\mathrm{Tr} F^n|_{H^1(X)} +\mathrm{Tr} F^n|_{H^2(X)}.

This gives the general form of the theorem.

The assertion on the absolute values of the αs requires some deeper argument.

The whole idea fits into the framework of motives: formally [X] = [point]+[line]+[1-part], and [1-part] has something like \sqrt p points.

[edit] External links

[edit] References

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