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.
1. The 180-Hz sinusoid is called the third harmonic, since its frequency is three times that of the fundamental frequency. Similarly, the peaky sinusoid in Fig. 2 has the mathematical equation, y=sin (x) -0.25 sin (3x).his waveform has the same composition as the first waveform, except the third harmonic component is out of phase with the fundamental frequency, as indicated by the negative sign preceding the â0.25 sin (3x)â term. This subtle mathematical difference produces a very different appearance in the waveform.