Published online by Cambridge University Press: 05 July 2011
Abstract
In the first four sections of this chapter we consider an ordinary differential equation of any order invariant under some nontrivial group G of local point transformations. We show how such an ODE can be approximated by a difference scheme invariant under the same group G. Some advantages of such invariant schemes are pointed out. The schemes are exact for first-order equations. They can be solved analytically for some second-order equations. Used for numerical calculations the invariant schemes provide better qualitative descriptions of solutions than standard methods, specially close to singularities. The last two sections are devoted to methods of determining the Lie point symmetries of differential difference equations on fixed nontransforming lattices.
Introduction
Lie group theory started out as a theory of continuous transformations in the space of independent and dependent variables figuring in a system of differential equations. These point transformations were so constructed as to leave the space of solutions invariant, i.e., transform solutions into solutions. After Sophus Lie's seminal work in the end of the 19th and beginning of the 20th century. Lie theory developed in several directions, one being abstract group theory, another applications. In particular Lie group theory has evolved into a very general and powerful tool for obtaining exact (analytic) solutions of large classes of ordinary and partial differential equations. The symmetry theory of differential equations has been reviewed in modern books and review articles [5, 6, 25, 35, 36, 69, 82].
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