### On Tate-Shafarevich groups of abelian varieties, by Cristian D. Gonzalez-Aviles

Let $K/F$ be a finite Galois extension of number fields with Galois group $G$, let $A$ be an abelian variety defined over $F$, and let ${\cyr W}(A_{^{/\! K}})$ and ${\cyr W}(A_{^{/\! F}})$ denote, respectively, the Tate-Shafarevich groups of $A$ over $K$ and of $A$ over $F$. Assuming that these groups are finite, we derive, under certain restrictions on $A$ and $K/F$, a formula for the order of the subgroup of ${\cyr W}(A_{^{/\! K}})$ of $G$-invariant elements. As a corollary, we obtain a simple formula relating the orders of ${\cyr W}(A_{^{/\! K}})$, ${\cyr W}(A_{^{/\! F}})$ and ${\cyr W}(A_{^{\,/\! F}}^{\chi})$ when $K/F$ is a quadratic extension and $A^{\chi}$ is the twist of $A$ by the non-trivial character $\chi$ of $G$.

Cristian D. Gonzalez-Aviles <cgonzale@abello.dic.uchile.cl>