Centrosymmetric matrix

Symmetry pattern of a centrosymmetric 5×5 matrix

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 = [A_i,j] is centrosymmetric when its entries satisfy

A_i,j = A_{n−i + 1,n−j + 1} for i, j ∊{1, ..., n}.

If J denotes the n×n exchange matrix with 1 on the antidiagonal and 0 elsewhere (that is, J_{i,n + 1 − i} = 1; J_i,j = 0 if j ≠ n +1− i), then a matrix A is centrosymmetric if and only if AJ = JA.

Examples[]

• All 2×2 centrosymmetric matrices have the form
  $${\displaystyle {\begin{bmatrix}a&b\\b&a\end{bmatrix}}.}$$

  
• All 3×3 centrosymmetric matrices have the form
  $${\displaystyle {\begin{bmatrix}a&b&c\\d&e&d\\c&b&a\end{bmatrix}}.}$$

  
• Symmetric Toeplitz matrices are centrosymmetric.

Algebraic structure and properties[]

• If A and B are centrosymmetric matrices over a field F, then so are A + B and cA for any c in F. Moreover, the matrix product AB 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 where J is the exchange matrix.
• If A is a centrosymmetric matrix with distinct eigenvalues, then the matrices that commute with A must be centrosymmetric.^[1]
• The maximum number of unique elements in a m × m centrosymmetric matrix is ${\displaystyle (m^{2}+m\%2)/2}$.

Related structures[]

An n×n matrix A is said to be skew-centrosymmetric if its entries satisfy A_i,j = −A_{n−i+1,n−j+1} for i, j ∊ {1, ..., n}. Equivalently, A is skew-centrosymmetric if AJ = −JA, where J is the exchange matrix defined above.

The centrosymmetric relation AJ = JA lends itself to a natural generalization, where J is replaced with an involutory matrix K (i.e., K² = I)^[2][3][4] or, more generally, a matrix K satisfying K^m = I for an integer m > 1.^[1] The inverse problem for the commutation relation AK = KA of identifying all involutory K that commute with a fixed matrix A has also been studied.^[1]

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.^[3] A similar result holds for Hermitian centrosymmetric and skew-centrosymmetric matrices.^[5]

References[]

Further reading[]

• Muir, Thomas (1960). A Treatise on the Theory of Determinants. Dover. p. 19. ISBN 0-486-60670-8.
• Weaver, James R. (1985). "Centrosymmetric (cross-symmetric) matrices, their basic properties, eigenvalues, and eigenvectors". American Mathematical Monthly. 92 (10): 711–717. doi:10.2307/2323222. JSTOR 2323222.

External links[]

• Centrosymmetric matrix on MathWorld.