The unit circle is an essential concept in mathematics, particularly in trigonometry. Understanding the unit circle makes it easier to grasp the foundation of many concepts, such as angles, sine, ...

J. D. Sargan, Alok Bhargava, Maximum Likelihood Estimation of Regression Models with First Order Moving Average Errors when the Root Lies on the Unit Circle, Econometrica, Vol. 51, No. 3 (May, 1983), ...