“…The tools which seem to be more promising are the Center Manifold Theory and the Normal Form Theory (see, for example, [22,167,61,134,114,140,115], where the two theories are connected to each other and to the Bifurcation Theory and to Chaos); the Tikhonov Theorem (see for example [158,47,67,76]); the Renormalization Group Theory applied to singular perturbations, recently rediscovered (see for example [58,26,45,77,116,133]); the Geometric Singular Perturbation Theory (GSPT) [52,68,78]. All these theories cope on one side with small perturbation parameters, on the other side with multiscale phenomena and timescale separation, which both allow the dimensional system reduction and the simplification of the model, according to the innovative ideas and techniques introduced and applied in several recent works by Luongo and coworkers [55,85,[87][88][89][90][91][92][93][94][95][96][97][98][99][100][101]123].…”