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

Wim Nijgh, Ronald van Luijk

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…