The generalized Nyquist stability criterion

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Once you have a MIMO matrix transfer function you can generate the plot by finding the eigenvalues of the matrix as a function of frequency. A 2x2 MIMO matrix transfer function will have 2 eigenvalues at every frequency point. The imaginary vs. the real part of the two eigenvalues will yield two loci that should always connect. This is the generalized Nyquist. I include a sample code for a textbook example in Multivariable Feedback Design by Maciejowski. Example 2.7. This example is limited to 2x2 but can be extended easily to higher dimensions.

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Thanks, prof. this is very helpful
Note that the 2017 file only plotted one eigen value, which was an oversight. This updated version Gnyquist2 plots both eigen values as it should and it's a bit faster. Note that in some cases the eigenvalues need to be sorted to maintain the continuity of the eign-loci. Otherwise the eign-values will trade places and it shows as a jump in the plot.

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Is it only applicable for square matrices?

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Yes but normally the open loop K*G(s) is always square for state and output feedback, also if G(s) is not.

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