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.
The a in the expression y = a sin x represents the amplitude of the graph. It is an indication of how much energy the wave contains. The amplitude is the distance from the resting position (otherwise known as the mean value or average value) of the curve.