If � is a square matrix, and if a matrix � of the same size can be found such that �� = �� = �, then � is said to be invertible (or nonsingular) and � is called an inverse of �. If no such matrix � can be found, then � is said to be singular.Remark 1: The relationship �� = �� = � is not changed by interchanging � and �, so if � is invertible and � is an inverse of �, then it is also true that � is invertible, and � is an inverse of B