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.
Sine waves can be measured too. The shape of a sine wave is given by its amplitude, phase, wavelength and frequency. The speed that the sine wave moves can be measured. The amplitude and wavelength of the sine wave is shown in the picture. The highest point on a wave is called the peak. The lowest point is called the trough. The peak of a wave and the trough of a wave are always twice the wave's amplitude apart from each other. The part of the wave half way in between the peak and the trough is called the baseline. Complicated wave forms (like the sound waves of music) can be made by adding up sine waves of different frequencies.