Nyquist, extended [Commentary on "Stability of Systems with Delayed Feedback" (1946)]

This paper by Tsypkin develops a major extension of the Nyquist criterion for systems with feedback delay. In recognition of this extension, the paper was given the place of honor immediately after the paper by Nyquist in a compendium of results on frequency-response methods in control systems edited by A.G.J. MacFarlane (Frequency-Response Methods in Control Systems, IEEE Press, New York, 1979).