A Nyquist-like test for the stability of two-dimensional digital filters

This paper constructs a Nyquist-like test for the stability of two-dimensional digital filters. The test takes the form of a continuum of classical one-variable Nyquist plots parameterized by the elements of the unit circle of the complex plane. Since the parameter space is compact, the test can be accurately approximated by a finite number of classical one-variable Nyquist plots and is therefore readily implemented on a computer.