“…Its particularly simple structure makes it a natural setting for testing and developing various mathematical and physical ideas, such as problems of fractal geometry, harmonic analysis, random walks and PDEs on fractals, networks, number theory, etc. ; see [1], [3]- [6], chapter 12 of [7], [8], [11], [13], [19]- [23], [35], [39] and the references therein. Its pattern of construction seems to occur naturally to the human mind, at least in artistic contexts.…”