“…Similar topological methods are also employed to a great extent for the existence of non-negative periodic solutions of degenerate and doubly degenerate parabolic equations, see [3], [9], [20], [30], [31], [38], [42], [44], [45], [48], [55], [56], [58], [59], [60], [61], [62], [63], [64], [67], [68]. Nonlocal models to study aggregation in biological systems with degenerate diffusion are proposed in several papers, see the recent [12], [43] and the references therein.…”