On the Galois-invariant part of the Weyl group of the Picard lattice of a K3 surface

Abstract

Let $X$ denote a K3 surface over an arbitrary field $k$. Let $k^{\text{s}}$ denote a separable closure of $k$ and let $X^{\text{s}}$ denote the base change of $X$ to $k^{\text{s}}$. The action of the absolute Galois group Gal($k^{\text{s}}/k$) of $k$ on Pic $X^{\text{s}}$ respects the intersection pairing, which gives Pic $X^{\text{s}}$ the structure of a lattice. Let O(Pic $X$) and O(Pic $X^{\text{s}}$) denote the group of isometries of Pic $X$ and Pic $X^{\text{s}}$, respectively. Let $R_X$ denote the Galois invariant part of the Weyl group of O(Pic $X^{\text{s}}$). One can show that each element in $R_X$ can be restricted to an element of O(Pic $X$). The following question arises: Is the image of the restriction map $R_X\to$O(Pic $X$) a normal subgroup of O(Pic $X$) for every K3 surface $X$? We show that the answer is negative by giving counterexamples over $k=\mathbb{Q}$.

12 pages