In the recent paper by Kudryashov [Commun. Nonlinear Sci. Numer. Simulat.,2009, V.14, 3507-3529] seven common errors in finding exact solutions ofnonlinear differential equations were listed and discussed in detail. Weindicate two more common errors concerning the similarity (equivalence withrespect to point transformations) and linearizability of differential equationsand then discuss the first of them. Classes of generalized KdV and mKdVequations with variable coefficients are used in order to clarify ourconclusions. We investigate admissible point transformations in classes ofgeneralized KdV equations, obtain the necessary and sufficient conditions ofsimilarity of such equations to the standard KdV and mKdV equations and carriedout the exhaustive group classification of a class of variable-coefficient KdVequations. Then a number of recent papers on such equations are commented usingthe above results. It is shown that exact solutions were constructed in thesepapers only for equations which are reduced by point transformations to thestandard KdV and mKdV equations. Therefore, exact solutions of such equationscan be obtained from known solutions of the standard KdV and mKdV equations inan easier way than by direct solving. The same statement is true for otherequations which are equivalent to well-known equations with respect to pointtransformations.
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