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.
Sinusoid is the general form of a sine function.In f(x) = A0 Sin [B(x-C)] + K ,| Ao |= Amplitude of the sine waveB = Number of cycles from 0 to 2$\pi$Period = 2$\pi$ / BK = Vertical ShiftC = Horizontal shift. The amplitude is half the distance between the highest and the lowest values of the function.