# Inverse of a Toeplitz matrix

A Toeplitz matrix or diagonal-constant matrix is a matrix in which each descending diagonal from left to right is constant. For instance, the following matrix is an $$n\times n$$ Toeplitz matrix:

$$A = \begin{bmatrix} a_{0} & a_{-1} & a_{-2} & \ldots & \ldots &a_{-n+1} \\\ a_{1} & a_0 & a_{-1} & \ddots & & \vdots \\ a_{2} & a_{1} & \ddots & \ddots & \ddots& \vdots \\\ \vdots & \ddots & \ddots & \ddots & a_{-1} & a_{-2}\\\ \vdots & & \ddots & a_{1} & a_{0}& a_{-1} \\ a_{n-1} & \ldots & \ldots & a_{2} & a_{1} & a_{0} \end{bmatrix}$$

I'm interested in the self-adjoint case ($$a_{-k}=a_{k}\in\mathbb{R}$$).

My questions are:

• Is there a relatively simple criterion to know when these matrices are invertible by just analyzing the sequence $$\{a_{0},\ldots,a_{n-1}\}$$?

• In the invertible case, what is known about its inverse?

Thanks!

• If $\mathbf A$ is symmetric positive definite and Toeplitz, then there is an $O(n^2)$ method due to Trench for inverting it. So the SPD case is easy at least; what I'm fuzzy with is if (stable) methods for the symmetric indefinite case have been developed. (There are $O(n\log n)$ methods based on FFT, but I have no experience with using them.) May 11 '11 at 12:53
• I have yet to read it, but this might be of use. See also this interesting letter by Trench. May 11 '11 at 13:34
• @Tom: Interesting. Your comment also raised a good question. What are the conditions on the sequence $\{a_0,\ldots,a_{n-1}\}$ for $A$ to be positive definite?
– ght
May 11 '11 at 14:19
• You can apply the Gershgorin circle theorem to get a sufficient result for $A$ to be invertible, namely that it is strictly diagonally dominant $$|a_0| > \sum_{i=1}^{n-1} |a_i|$$ If this is true, a sufficient condition for it to be positive definite is that $a_0>0$. May 11 '11 at 18:24
• Crossposted to MO. Some patience would be good for you, you know. May 11 '11 at 18:32

About the last point of you question I think it's not really simple to state a closed simple formula for the determinants. I tried to see if there are symmetries. There are but not really useful (at least as far as I can see).

Just to have an idea the first 3 steps of the induction you have the following determinants.

If $$n=2$$ then

$$$$\det A^{(2)} = a_0^2 - a_1^2$$$$

If $$n=3$$ then

$$$$\det A^{(3)} = (a_0 - a_2) (a_0^2 - 2 a_1^2 + a_0 a_2)$$$$

Already at $$n=4$$ the formula is not so simple. Indeed if we have $$n=4$$ we the determinant become:

$$$$\det A^{(4)} = a_0^4 + a_1^4 + a_2^4 - 2 a_1 a_2^2 a_3 +4 a_0 a_1^2 a_2 - 3 a_1^2a_0^2 -2 a_2^2a_1^2 - 2 a_2^2a_0^2 + a_3^2a_1^2 - a_3^2a_0^2 - 2 a_1^3 a_3 + 4 a_0 a_1 a_2 a_3$$$$

What happens is that when you have to calculate $$\det A^{(n)}$$ the minor determinants of order $$n-1$$ are matrices where the condition of symmetry that allowed huge simplifications disappears. With n=2,3 the problem is not so big since the minors are trivial, but when n gets bigger the problems arise. In fact the minors are not really Toeplitz matrix, but "block Toeplitz Matriz" (sort of saying). So maybe there could be a way of enclosing the writing in a simple notation formula, but it wouldn't be a real computational gain...