Isometries of quadratic spaces

Abstract

Let k be a field of characteristic not 2, let q be a quadratic space over k and let f be an irreducible polynomial with coefficients in k. In 1969, Milnor raised the following question : how can we decide whether q has an isometry with minimal polynomial f ? We give an answer to this question in the case of global fields. A more general version of the question is also considered.