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 Fourier Transform is the mathematical tool that shows us how to deconstruct the waveform into its sinusoidal components. This has a multitude of applications, aides in the understanding of the universe, and just makes life much easier for the practicing engineer or scientist.