“…To process such signals, one needs to extend the well-developed theory of classical signal processing to graph signals. There have been a lot of researches on graph signal processing, including graph shift operators [4,5,6,7,8,9,10,11,12,13], graph filters [14,15,16,17,18], graph Fourier transforms [19,20,21], windowed graph Fourier transforms [4,22], graph wavelets [23,24,25,26,27], graph signal sampling [28,29,30,31,32,33,34,35,36,37], multiscale analysis [38,39,40], and approximation theory for graph signals [41,42,43].…”