For Hermitian congruence , seer Conjuctivity . Congruence implies equivalence. Congruence is an equivalence relation. U H U for some non-singular upper triangular U . It is also known as the direct product or tensor product of A and B .
Similar matrices represent the same linear transformation in a different basis.
Similarity implies equivalence . Similar matrices have the same trace, determinant, eigenvalues and characteristic equation. Unitary similarity implies both similarity and conjunctivity . If A and B are real, they are orthogonally similar . Orthogonal similarity implies both similarity and congruence .