A set that is not convex is called a non-convex set. A polygon that is not a convex polygon is sometimes called a concave polygon, and some sources more generally use the term concave set to mean a non-convex set, but most authorities prohibit this usage.
In either case you don't need to deal with the absolute value. A rectified sine wave is a periodic signal with a period equal to half of the full sinusoid, I would write the sine in exponential form and integrate over one period to find the series coefficients.
sine wave. n. A waveform with deviation that can be graphically expressed as the sine curve. sine wave. n. (General Physics) any oscillation, such as a sound wave or alternating current, whose waveform is that of a sine curve. sineâ² wave`. n.