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$a Kp. Applications of Lie groups to differential equations / Peter J. Olver. - 2nd ed. - New York, 1993.
670
$a Kp. Group theoretic methods in bifurcation theory / D. H. Sattinger ; with an appendix by P. Olver. - Berlin, 1979. $b (School of Mathematics, Univ. of Minnesota, Minneapolis)