We show that if a field k contains sufficiently many elements (for instance, if k is infinite), and K is an algebraically closed field containing k, then every linear algebraic k-group over K is k-isomorphic to Aut(A ⊗k K), where A is a finite dimensional simple algebra over k. 1. Introduction In this paper, ‘algebra’ over a field means ‘nonassociative algebra’, ., a vector space A over this field with multiplication given by a linear map A ⊗ A → A, a1 ⊗ a2 → a1 a2 , subject to no a priori conditions; cf. .
