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Complex Methods for Lie Symmetry Analysis

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Symmetries and Applications of Differential Equations

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Abstract

When Lie developed symmetry analysis, he took the equations to be defined in the complex domain but did not explicitly use the entailed complex analyticity. Making it explicit necessitates the incorporation of the Cauchy–Riemann equations into the original system of equations, which modifies the symmetries of the system. This point was followed up by us, and some of our students, in a series of papers (and theses). It was found that complex methods, when they are applicable, provide more powerful tools for obtaining solutions and integrals of differential equations, even enabling us to find solutions of systems of differential equations that possess no symmetries. In this chapter we review the methods developed and then pose the crucial question that was begged in saying “when they are applicable.” When would they be applicable and why, or how, does the complex method work? We indicate some lines to pursue to try to find the answers, or at least partial answers, to these questions.

Dedicated to the memory of one of the most innovative workers in the field of Symmetry Analysis, after Sophus Lie, Nail Hairullovich Ibragimov, who initiated many new methods for using Lie Analysis to deal with differential equations.

FM is Visiting Professor at UNSW for 2020.

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Acknowledgements

We are grateful to Rimsha Khalil for pointing out some typographical errors in the chapter. FMM is grateful to the NRF of South Africa for support.

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Qadir, A., Mahomed, F.M. (2021). Complex Methods for Lie Symmetry Analysis. In: Luo, A.C.J., Gazizov, R.K. (eds) Symmetries and Applications of Differential Equations. Nonlinear Physical Science. Springer, Singapore. https://doi.org/10.1007/978-981-16-4683-6_4

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