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.
A violent oscillation or vibration The discomfort caused by the bat's reverberation surprised Tommy. An echo, or a series of overlapping echos The reverberation that followed Marylin's shout filled the cavern. The reflection of light or heat; a reflection in, or as though in, a mirror. Like the several reverberations of the same image from two opposite looking glasses. (plural) An evolving series of effects resulting from a particular event; a repercussion Reverberations from the Vietnam war affect our society to this day.