“…Although we have restricted our investigation to ordinary scalar Schrödinger operators, the concepts of ordinary, two-step, and multi-step SI are applicable to much wider systems. In fact, N -fold SUSY was successfully formulated also for Schrödinger operators with position-dependent mass [35], matrix ones [41], and ones with reflection operators [42]. Hence, we would be able to make systematic studies on various SI in these systems with the framework of N -fold SUSY, as have been done in this work.…”