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.
Rectified means the edges are ground true and that does nothing for the flatness. That type of tile even though it may be high quality, must have a grout joint to hide the un-eveness of the surface or you have to be able to live with a certain amount of lippage.