The Kac–Murdock–Szegö matrix is the symmetric Toeplitz matrix

It was considered by Kac, Murdock, and Szegö (1953), who investigated its spectral properties. It arises in the autoregressive AR(1) model in statistics and signal processing.

The matrix is singular for , as is the rank- matrix , and it is also rank- for , as in this case every column is a multiple of the vector with alternating elements . The determinant . For , is nonsingular and the inverse is the tridiagonal (but not Toeplitz) matrix

For , is positive definite, since every leading principal submatrix has positive determinant, as can also be seen by noting that the inverse is diagonally dominant with positive diagonal, so that is positive definite and hence is positive definite.

For , is positive semidefinite, so it is a correlation matrix for in this range.

For , is totally nonnegative, that is. every submatrix has nonnegative determinant. For , we know that is nonsingular, and it is clearly irreducible, and together with the total nonnegativity these properties imply that the eigenvalues are distinct and positive (this can also be deduced from the fact that the inverse is tridiagonal with nonzero subdiagonal and superdiagonal entries).

It is straightforward to verify that has a factorization with the inverse of a unit lower bidiagonal matrix:

This factorization can be used to prove all the properties stated above.

From (1) and (2) we can derive the formulas

Hence we have an explicit formula for the condition number for .

We can allow to be complex, in which case the definition (1) is modified to conjugate the elements below the diagonal. The factorization continues to hold with in replaced by .

The Kac–Murdock–Szegö matrix (for real or complex ) can be generated in MATLAB as `gallery('kms',n,rho)`

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## References

This is a minimal set of references, which contain further useful references within.

- George Fikioris, Spectral Properties of Kac–Murdock-Szegö Matrices with a Complex Parameter, Linear Algebra Appl 553, 182–210, 2018.
- M. Kac, W. L. Murdock, and G. Szegö, On the Eigen-values of Certain Hermitian Forms, Journal of Rational Mechanics and Analysis 2, 767–800, 1953.

## Related Blog Posts

- What Is a Condition Number? (2020)
- What Is a Correlation Matrix? (2020)

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