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.
Waves Math Images : Mathematicians use the sine function (Sine) to express the shape of a wave. Particle Sizing by Static Laser Light Scattering : The Effect of Resolution on the Capability to Accurately Represent the Scattering Pattern as a Set of angle . wave most often being depicted as sinusoidal.