“…The criterion of "nonclassical" invariance was firstly formulated in [19] and the rigorous theory of the nonclassical reduction method, theory of reduction modules, was recently developed in [6]. The nonclassical reduction operators are also called nonclassical symmetries [36], conditional symmetries [29] and Q-conditional symmetries [17] (see the related discussion in [28] and some more research papers of interest [14,15,21,33,37,62]).…”