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 problem statement, all variables and given/known data. Derive the FT for a full-wave rectified sine wave, i.e., |sin(wt)|. 2. Relevant equations. $$1/(â2Ï)\int_{a}^{b} |Sin[wt]| {e}^{-i w t}dt$$. 3. The attempt at a solution. I'm not entirely sure how to start doing this problem.