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.
rectifier (rekÂ´tifÄ«ur) , n a device used for converting an alternating current to a direct current; it also prevents or limits the flow of current in the opposite direction. rectifier, full-wave, n an apparatus for rectifying the entire wave of an alternating current in a radiography machine by means of a mechanical rectifier or valve tube. rectifier, half-wave, n an apparatus used in the rectifying of half of the sine wave in a radiography unit.