Tate–Shafarevich group

In arithmetic geometry, the Tate–Shafarevich group Ш(A/K) of an abelian variety A (or more generally a group scheme) defined over a number field K consists of the elements of the Weil–Châtelet group

W

C

(

A

/

K

)

=

H

1

(

G

K

,

A

)

{\displaystyle \mathrm {WC} (A/K)=H^{1}(G_{K},A)}

, where

G

K

=

G

a

l

(

K

a

l

g

/

K

)

{\displaystyle G_{K}=\mathrm {Gal} (K^{alg}/K)}

is the absolute Galois group of K, that become trivial in all of the completions of K (i.e., the real and complex completions as well as the p-adic fields obtained from K by completing with respect to all its Archimedean and non Archimedean valuations v). Thus, in terms of Galois cohomology, Ш(A/K) can be defined as

⋂

v

k

e

r

(

H

1

(

G

K

,

A

)

→

H

1

(

G

K

v

,

A

v

)

)

.

{\displaystyle \bigcap _{v}\mathrm {ker} \left(H^{1}\left(G_{K},A\right)\rightarrow H^{1}\left(G_{K_{v}},A_{v}\right)\right).}

This group was introduced by Serge Lang and John Tate and Igor Shafarevich. Cassels introduced the notation Ш(A/K), where Ш is the Cyrillic letter "Sha", for Shafarevich, replacing the older notation TS or TŠ.

Elements of the Tate–Shafarevich group

Geometrically, the non-trivial elements of the Tate–Shafarevich group can be thought of as the homogeneous spaces of A that have Kv-rational points for every place v of K, but no K-rational point. Thus, the group measures the extent to which the Hasse principle fails to hold for rational equations with coefficients in the field K. Carl-Erik Lind gave an example of such a homogeneous space, by showing that the genus 1 curve x4 − 17 = 2y2 has solutions over the reals and over all p-adic fields, but has no rational points.

Ernst S. Selmer gave many more examples, such as 3x3 + 4y3 + 5z3 = 0.

The special case of the Tate–Shafarevich group for the finite group scheme consisting of points of some given finite order n of an abelian variety is closely related to the Selmer group.

Tate–Shafarevich conjecture

The Tate–Shafarevich conjecture states that the Tate–Shafarevich group is finite. Karl Rubin proved this for some elliptic curves of rank at most 1 with complex multiplication. Victor A. Kolyvagin extended this to modular elliptic curves over the rationals of analytic rank at most 1.