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.
Another common sinusoidal waveform used in synthesis or measurement is the cosine wave, which is exactly the same shape as a sine wave and would sound exactly the same all by itself, but is distinguished by the fact that its cycle begins 90Â° out of phase to a sine wave, or at +1. ___________________. 1 Dodge p. 29-32.