 Matrix congruence

Two square matrices $A,C$ are congruent when $C=P^{T}AP$ for an invertible matrix $P$. This is the transformation of the matrix of a bilinear form under change of basis, and also transforms a quadratic form under an invertible linear map on its coordinates. The matrices need not be symmetric. It differs from matrix similarity, which uses $P^{-1}AP$ for a linear operator. For real symmetric matrices, completing the square diagonalizes the corresponding quadratic form by matrix congruence.

Matrix congruence is a concept in linear algebra that relates to two matrices being similar in a specific way through the use of a non-singular matrix. Specifically, two square matrices \( A \) and \( B \) are said to be congruent if there exists a non-singular matrix \( P \) such that: \[ A = P^T B P \] Here, \( P^T \) denotes the transpose of the matrix \( P \).