In mathematics, especially in linear algebra and matrix theory, a centrosymmetric matrix is a matrix which is symmetric about its center. More precisely, an n × n matrix A = is centrosymmetric when its entries satisfy If J denotes the n × n matrix with 1 on the counterdiagonal and 0 elsewhere, then a matrix A is centrosymmetric if and only ifAJ = JA. The matrix J is sometimes referred to as the exchange matrix.
If A and B are centrosymmetric matrices over a given fieldF, then so are A+B and cA for any c in F. In addition, the matrix productAB is centrosymmetric, since JAB = AJB = ABJ. Since the identity matrix is also centrosymmetric, it follows that the set of n × n centrosymmetric matrices over F is a subalgebra of the associative algebra of all n × n matrices.
If A is a centrosymmetric matrix with an m-dimensional eigenbasis, then its m eigenvectors can each be chosen so that they satisfy either x=Jx or x=-Jx.
If A is a centrosymmetric matrix with distincteigenvalues, then the matrices that commute with A must be centrosymmetric.
An n × n matrix A is said to be skew-centrosymmetric if its entries satisfy Ai,j = -An−i+1,n−j+1 for 1 ≤ i,j ≤ n. Equivalently, A is skew-centrosymmetric if AJ = -JA, where J is the exchange matrix defined above. The centrosymmetric relation AJ = JAlends itself to a natural generalization, where J is replaced with an involutory matrixK or, more generally, a matrix K satisfying Km = I for an integer m > 1. The inverse problem for the commutation relationAK = KA of identifying all involutory K that commute with a fixed matrix A, has also been studied. Symmetric centrosymmetric matrices are sometimes called bisymmetric matrices. When the ground field is the field of real numbers, it has been shown that bisymmetric matrices are precisely those symmetric matrices whose eigenvalues remain the same aside from possible sign changes following pre or post multiplication by the exchange matrix. A similar result holds for Hermitian centrosymmetric and skew-centrosymmetric matrices.
