A Nyquist-type stability test for multivariable distributed systems

A multivariable Nyquist criterion is given that applies to a wide class of open-loop transfer matrices, including some that are improper or have infinitely many poles in the right-half plane. Application of the test to specific examples requires only elementary knowledge of complex variable theory. The actual test requires operations similar to those used in the standard Nyquist test. The simplicity of the frequency-domain description of the class T (analyticity and polynomial boundedness) makes it relatively easy to verify if the test is applicable to a given transfer matrix.<
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