Artin–Verdier duality

In mathematics, Artin–Verdier duality is a duality theorem for constructible abelian sheaves over the spectrum of a ring of algebraic numbers, introduced by Artin and Verdier (1964), that generalizes Tate duality.

It shows that, as far as etale (or flat) cohomology is concerned, the ring of integers in a number field behaves like a 3-dimensional mathematical object.

Statement

Let X be the spectrum of the ring of integers in a totally imaginary number field K, and F a constructible etale abelian sheaf on X. Then the Yoneda pairing

$H^r(X,F)\times Ext^{3-r}(F,{\mathbb G}_m)\longrightarrow H^3(X,{\mathbb G}_m)={\mathbb Q}/{\mathbb Z}$

is a non-degenerate pairing of finite abelian groups, for every integer r.

Here, H ^r(X,F) is the r-th etale cohomology group of the scheme X with values in F, and Ext ^r(F,G) is the group of r-extensions of the etale sheaf G by the etale sheaf F in the category of etale abelian sheaves on X. Moreover, G_m denotes the etale sheaf of units in the structure sheaf of X.

Finite flat group schemes

Let U be an open subscheme of the spectrum of the ring of integers in a number field K, and F a finite flat commutative group scheme over U. Then the cup product defines a non-degenerate pairing

$H^r(U,F^D)\times H_c^{3-r}(U,F)\longrightarrow H_c^3(U,{\mathbb G}_m)={\mathbb Q}/{\mathbb Z}$

of finite abelian groups, for all integers r.

Here F ^D denotes the Cartier dual of F, which is another finite flat commutative group scheme over U. Moreover, H ^r(U,F) is the r-th flat cohomology group of the scheme U with values in the flat abelian sheaf F, and H_c^r(U,F) is the r-th flat cohomology with compact supports of U with values in the flat abelian sheaf F.

The flat cohomology with compact supports is defined to give rise to a long exact sequence

$\ldots\longrightarrow H^r_c(U,F)\longrightarrow H^r(U,F)\longrightarrow \bigoplus_{v\notin U} H^r(K_v,F)\longrightarrow H^{r+1}_c(U,F)\longrightarrow\ldots$ .

The sum is taken over all places of K, which are not in U, including the archimedean ones. The local contribution H ^r(K_v ,F) is the Galois cohomology of the Henselization K_v of K at the place v, modified a la Tate:

$H^r(K_v,F)=H^r_T(\mathrm{Gal}(K_v^s/K_v),F(K_v^s))$ .

Here K_v^s is a separable closure of K_v .