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Persymmetric matrix

Square matrix symmetric about its anti-diagonal From Wikipedia, the free encyclopedia

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In mathematics, persymmetric matrix may refer to:

  1. a square matrix which is symmetric with respect to the northeast-to-southwest diagonal (anti-diagonal); or
  2. a square matrix such that the values on each line perpendicular to the main diagonal are the same for a given line.

The first definition is the most common in the recent literature. The designation "Hankel matrix" is often used for matrices satisfying the property in the second definition.

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Definition 1

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Symmetry pattern of a persymmetric 5×5 matrix

Let A = (aij) be an n×n matrix. The first definition of persymmetric requires that for all i, j.[1] For example, 5×5 persymmetric matrices are of the form

This can be equivalently expressed as AJ = JAT where J is the exchange matrix.

A third way to express this is seen by post-multiplying AJ = JAT with J on both sides, showing that AT rotated 180 degrees is identical to A:

A symmetric matrix is a matrix whose values are symmetric in the northwest-to-southeast diagonal. If a symmetric matrix is rotated by 90°, it becomes a persymmetric matrix. Symmetric persymmetric matrices are sometimes called bisymmetric matrices.

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Definition 2

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The second definition is due to Thomas Muir.[2] It says that the square matrix A = (aij) is persymmetric if aij depends only on i + j. Persymmetric matrices in this sense, or Hankel matrices as they are often called, are of the form A persymmetric determinant is the determinant of a persymmetric matrix.[2]

A matrix for which the values on each line parallel to the main diagonal are constant is called a Toeplitz matrix.

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