(Taylor and Francis Group, 2023-05-16) Borobia Vizmanos, Alberto; Canogar Mckenzie, Roberto; Teran, Fernando De; Ministerio de Ciencia e Innovación de España
We provide a necessary and su cient condition for the matrix equation XTAX B to be consistent, when A is an arbitrary complex square matrix and B is skew-symmetric. This problem is equivalent to nd the largest dimension of a subspace in which the bilinear form A is symplectic. The necessity is valid for any A and B as above, whereas the su ciency is proved to be valid for any skew-symmetric matrix B and for all complex square matrices A whose Canonical form for congruence (CFC) does not contain blocks 0 1 1 1 . The provided condition improves the one in [A. Borobia, R. Canogar, F. De Teran, Lin. Multilin. Algebra, 2022. doi:10.1080/03081087.2022.2093825], because it includes the case where CFC(A) includes symmetric blocks, and it is given in terms of the size of A and the rank of its symmetric and skew-symmetric parts. More precisely, if A is n n, we prove that the equation is consistent if and only if rankB mintn nA rankpA ATq 2 NulAXNulAT.
