Nyquist plot [SEP] named after
using polar coordinates, where gain of the transfer function is the radial coordinate, and the phase of the transfer function is the corresponding angular coordinate. The Nyquist plot is named after Harry Nyquist, a former engineer at Bell Laboratories.
Assessment of the stability of a closed-loop negative feedback system is done by applying the Nyquist stability criterion to the Nyquist plot of the open-loop system (i.e. the same system without its feedback loop). This method is easily applicable even for systems with delays and other non-
equation are also the zeros of formula_17, or simply the roots of formula_18.
Cauchy's argument principle.
From complex analysis, a contour formula_19 drawn in the complex formula_20 plane, encompassing but not passing through any number of zeros and poles of a function formula_21, can be mapped to another plane (named formula_21plane) by the function formula_23. Precisely, each complex point formula_20 in the contour formula_19 is mapped to the point formula_21 in the new formula_21 plane yielding a new contour. 
The Nyquist plot of formula_21,