I am reading my textbook and have been trying to figure this out all night and day, why is a = r +jw0 and what is r? The book described this as "a in rectangular form" but I was under the impression rectangular form specified a real x component and an imaginary y component similar to cartesian fashion. I understand that r is the real part and jw0 is the imaginary part but can't quite grasp why we used the fundamental angular velocity instead of a "y" value or click on the context and how it relates to the periodic complex exponential e^jw0t. Any help is appreciated, I've moved on but would like to fundamentally understand how this is derived.