 Nyquist stability criterion

Let $L$ be the open-loop transfer function. The winding of its Nyquist plot around $-1$, together with the number of unstable open-loop poles, determines the number of unstable zeros of $1+L$, which are the closed-loop poles.

The Nyquist stability criterion is a fundamental principle in control theory used to determine the stability of a linear time-invariant (LTI) system based on its frequency response. Specifically, it relates the open-loop frequency response of a system to the stability of the closed-loop system.