Simultaneous orthogonalization of inner products in infinite-dimensional vector spaces

Abstract

For an arbitrary field K and a family of inner products in a K-vector space V of arbitrary dimension, we study necessary and sufficient conditions in order to have a basis which is orthogonal relative to all the inner products. If the family contains a nondegenerate element plus a compatibility condition, then under mild hypotheses the simultaneous orthogonalization can be achieved. So we investigate several constructions whose purpose is to add a nondegenerate element to a degenerate family and we study under what conditions the enlarged family is nondegenerate.