Stability testing of 2-d discrete linear systems by telepolation of an immittance-type tabular test

A new procedure for deciding whether a bivariate (two-dimensional, 2-D) polynomial with real or complex coefficients does not vanish in the closed exterior of the unit bi-circle (is "2-D stable") is presented. It simplifies a recent immittance-type tabular stability test for 2-D discrete-time systems that creates for a polynomial of degree (n ₁, n ₂) a sequence of n ₂ (or n ₁) centro-symmetric 2-D polynomials (the "2-D table") and requires the testing of only one last one dimensional (1-D) symmetric polynomial of degree 2n ₁n ₂ for no zeros on the unit circle. It is shown that it is possible to bring forth (to "telescope") the last polynomials by interpolation without the construction of the 2-D table. The new 2-D stability test requires an apparently unprecedentedly low count of arithmetic operations. It also shows that stability of a 2-D polynomial of degree (n ₁, n ₂) is completely determined by n ₁n ₂ + 1 stability tests (of specific form) of 1-D polynomials of degrees n ₁ or n ₂ for the real case (or 2n ₁ n ₂ + 1 polynomials in the complex cases).