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.
In fact, ALL signals can be created by combining sine wave. Sine Wave = Frequency. Cycle = One repetition of a wave's pattern. Frequency = The number of cycles per second (measured in Hz) Period = The time duration of one cycle (the inverse of frequency, P = 1/f ) Wavelength = The length of one period of a wave.