Let $ G $ be a connected reductive algebraic group over a field $ k $. We study the group of semilinear automorphisms Aut($ G\to $Spec $k$) consisting of algebraic automorphisms of $ G $ over automorphisms of $ k $. We focus on the exact sequence $ 1\to $Aut $G\to $Aut ($ G\to $Spec $k$)$\to $Aut$_{ G}(k)\to 1 $. When $G$ is quasi-split, we show that Aut$_{ G}(k)$ is isomorphic to Aut$_{ \mathcal{ R}(G)}(k)$, where $\mathcal{ R}(G)$ denotes the scheme of based root datum of $G$. Furthermore, the exact sequence $ 1\to $Aut $G\to $Aut ($ G\to $Spec $k$)$\to $Aut$_{ G}(k)\to 1 $ splits if and only if the exact sequence $ 1\to \text{ Aut }\mathcal{ R}(G) \to \text{ Aut }(\mathcal{ R}(G) \to \text{ Spec } k)\to \text{ Aut}_{ \mathcal{ R}(G)}(k)\to 1 $ splits. As a corollary, we get many examples of algebraic groups $ G $ over $ k $ whose group of abstract automorphisms does not decompose as the semidirect product of $ \text{ Aut } G $ with $\text{ Aut}_G(k) $. We also study the same questions for inner forms of SL$_n $ over a local field.
Categories: math.GR
