Integrable Systems (MAGIC083)
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This course is part of the MAGIC core.
The course is an introduction to the theory of integrable systems. We will consider mainly the infinite-dimensional systems such as nonlinear partial differential equations, differential-difference and partial difference equations. By integrability we understand the existence of an infinite hierarchy of symmetries and/or conservation laws. Lax representations are sufficient conditions for integrability. Corresponding Darboux transformations provide a link between integrable partial differential, differential-difference and partial difference equations. They enable us to construct exact multisoliton solutions, hierarchies of symmetries and conservation laws, as well as recursion operators. We will derive necessary conditions for integrability and apply them to the problem of classification of integrable systems. Main examples include nonlinear Schrödinger type equations, Volterra and Toda lattices, partial difference Boussinesq and Tzitzeica type equations. The major part of the course is based on well established theory, although some open yet unsolved problems and possible directions of research will also be presented.
Spring 2020 (Monday, January 20 to Friday, March 27)
Linear algebra and some elementary calculus. Excellent companions to the course Integrable Systems MAGIC083 are the MAGIC courses: Integrable Systems MAGIC067, Lie Groups and Lie Algebras MAGIC008, Nonlinear Waves MAGIC021.
Systems of ordinary differential equations, vector fields, first integrals, symmetries. Theorem of S.Lie on integration in quadratures. Partial differential equations, vector fields, symmetries, local conservation laws. Recursion operator. Symmetry reductions. Examples: KdV, NLS. Lax representations for PDEs. Derivation of hierarchies of conservation laws and symmetries. Construction of the recursion operator. Construction of exact "soliton" solutions, Darboux and Bäcklund transformations. Example: NLS. A chain of Bäcklund transformations as an integrable differential-difference system. Symmetries and local conservation laws of differential-difference systems. Example:Toda lattice. Bianchi commutativity of Darboux transformations and integrable systems of partial difference equations. Symmetries and local conservation laws of partial difference equations. Example: NLS. Formal pseudo-differential series residues and Adler's Theorem. Symmetries and/or conservation laws imply the existence of a formal recursion operator. Canonical conservation laws as integrability conditions for PDEs. Example: simple classification problem. Generalisation to differential-Difference and partial difference cases. Integrability conditions. Examples:Volterra lattice, partial difference Boussinesq and Tzitzeica type equations. BOOKS:  Ablowitz, M.J. Clarkson P.A. 1991 Solitons, Nonlinear Evolution Equations and Inverse Scattering, CUP.  Ablowitz, M.J. and Segur, H. 1981 Solitons and the Inverse Scattering Transform, SIAM.  Dodd, R.K., Eilbeck, J.C., Gibbon, J.D. and Morris, H.C. 1982 Solitons and Nonlinear Waves Equations, Academic Press, Inc.  Mikhailov, A.V. (Ed) 2009 Integrability, Springer.  Novikov, S.P., Manakov, S.V., Pitaevskii, L.P. and Zakharov, V.E. 1984 The Theory of Solitons: The Inverse Scattering Method, Consultants, New York.  Newell, A.C. 1985 Solitons in Mathematics and Physics, SIAM.  Zakharov, V.E.(Ed) 1991 What is Integrability? Springer.
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