Stability and margin of stability tests for multidimensional filters

In this paper we give simplified proofs of the stability and the margin of stability criteria of Zeheb and Walach for n-dimensional recursive digital filters. We base our proofs on the method of Lagrange multipliers. Both the stability and margin of stability questions are converted into the problem of locating the zeros of a system of polynomial equations. These systems can then be solved using Buchberger's algorithm for computing the Groebner basis of a polynomial ideal.