Congruence of Hermitian matrices by Hermitian matrices

Abstract

Two Hermitian matrices A , B ∈ M n ( C ) are said to be Hermitian-congruent if there exists a nonsingular Hermitian matrix C ∈ M n ( C ) such that B = CAC . In this paper, we give necessary and sufficient conditions for two nonsingular simultaneously unitarily diagonalizable Hermitian matrices A and B to be Hermitian-congruent. Moreover, when A and B are Hermitian-congruent, we describe the possible inertias of the Hermitian matrices C that carry the congruence. We also give necessary and sufficient conditions for any 2-by-2 nonsingular Hermitian matrices to be Hermitian-congruent. In both of the studied cases, we show that if A and B are real and Hermitian-congruent, then they are congruent by a real symmetric matrix. Finally we note that if A and B are 2-by-2 nonsingular real symmetric matrices having the same sign pattern, then there is always a real symmetric matrix C satisfying B = CAC . Moreover, if both matrices are positive, then C can be picked with arbitrary inertia.