On some algebraic problems in connection with general elgenvalue algorithms

Two real matrices A,B are S-congruent if there is a nonsingular upper triangular matrix R such that A = R^TBR. This congruence relation is studied in the set of all nonsingular symmetric and that of all skew-symmetric matrices. Invariants and systems of representation are give. The results are applied to the question of decomposability of a matrix in a product of an isometry and an upper triangular matrix, a problem crucial in eigenvalue algorithms.