Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. In mathematics, more specifically in the study of dynamical systems and differential equations, a Liénard equation is a certain type of differential equation, named after the French physicist Alfred-Marie Liénard. During the development of radio and vacuum tubes, Liénard equations were intensely studied as they can be used to model oscillating circuits. Under certain additional assumptions Liénard''s theorem guarantees the existence of a limit cycle for such a system.