Non-Hermitian matrices of even order and neutral subspaces of half the dimension

Abstract

Consider the sesquilinearmatrix equation X*DX + AX + X*B + C = 0, where all the matrices are square and have the same order n. With this equation, we associate a block matrix M of double order 2n. The solvability of the above equation turns out to be related to the existence of n-dimensional neutral subspaces for the matrix M. We indicate sufficiently general conditions ensuring the existence of such subspaces.