Doctor of Philosophy (PhD). / The term `Lax pair' refers to linear systems (of various types) that are related to nonlinear equations through a compatibility condition. If a nonlinear equation possesses a Lax pair, then the Lax pair may be used to gather information about the behaviour of the solutions to the nonlinear equation. Conserved quantities, asymptotics and even explicit solutions to the nonlinear equation, amongst other information, can be calculated using a Lax pair. Importantly, the existence of a Lax pair is a signature of integrability of the associated nonlinear equation. While Lax pairs were originally devised in the context of continuous equations, Lax pairs for discrete integrable systems have risen to prominence over the last three decades or so and this thesis focuses entirely on discrete equations. Famous continuous systems such as the Korteweg de Vries equation and the Painleve equations all have integrable discrete analogues, which retrieve the original systems in the continuous limit. Links between the different types of integrable systems are well known, such as reductions from partial difference equations to ordinary difference equations. Infinite hierarchies of integrable equations can be constructed where each equation is related to adjacent members of the hierarchy and the order of the equations can be increased arbitrarily. After a literature review, the original material in this thesis is instigated by a completeness study that finds all possible Lax pairs of a certain type, including one for the lattice modified Korteweg de Vries equation. The lattice modified Korteweg de Vries equation is subsequently reduced to several q-discrete Painleve equations, and the reductions are used to form Lax pairs for those equations. The series of reductions suggests the presence of a hierarchy of equations, where each equation is obtained by applying a recursion relation to an earlier member of the hierarchy, this is confirmed using expansions within the Lax pairs for the q-Painleve equations. Lastly, some explorations are included into fake Lax pairs, as well as sets of equivalent nonlinear equations with similar Lax pairs.

Doctor of Philosophy (PhD). / The term `Lax pair' refers to linear systems (of various types) that are related to nonlinear equations through a compatibility condition. If a nonlinear equation possesses a Lax pair, then the Lax pair may be used to gather information about the behaviour of the solutions to the nonlinear equation. Conserved quantities, asymptotics and even explicit solutions to the nonlinear equation, amongst other information, can be calculated using a Lax pair. Importantly, the existence of a Lax pair is a signature of integrability of the associated nonlinear equation. While Lax pairs were originally devised in the context of continuous equations, Lax pairs for discrete integrable systems have risen to prominence over the last three decades or so and this thesis focuses entirely on discrete equations. Famous continuous systems such as the Korteweg de Vries equation and the Painleve equations all have integrable discrete analogues, which retrieve the original systems in the continuous limit. Links between the different types of integrable systems are well known, such as reductions from partial difference equations to ordinary difference equations. Infinite hierarchies of integrable equations can be constructed where each equation is related to adjacent members of the hierarchy and the order of the equations can be increased arbitrarily. After a literature review, the original material in this thesis is instigated by a completeness study that finds all possible Lax pairs of a certain type, including one for the lattice modified Korteweg de Vries equation. The lattice modified Korteweg de Vries equation is subsequently reduced to several q-discrete Painleve equations, and the reductions are used to form Lax pairs for those equations. The series of reductions suggests the presence of a hierarchy of equations, where each equation is obtained by applying a recursion relation to an earlier member of the hierarchy, this is confirmed using expansions within the Lax pairs for the q-Painleve equations. Lastly, some explorations are included into fake Lax pairs, as well as sets of equivalent nonlinear equations with similar Lax pairs.

This dissertation deals with a class of nonlinear wave equations of the type discovered by R. Camassa and D. D. Holm which includes the Camassa-Holm, the Degasperis-Procesi, and the two component Camassa-Holm equations. All these equations admit certain non-smooth soliton-like solutions, called peakons as well as other non-smooth solutions like cuspons. We apply the techniques of the theory of distributions of L. Schwartz to study these solutions. In particular, every non-smooth traveling wave which is a distributional solution of the two component Camassa-Holm equation is a distributional solution of the Camassa-Holm equation if the set of points where the height of the wave equals its speed, is of measure zero. This includes peakon or cuspon traveling wave solutions.<p>We also develop a suitable modification of the classical Lax pair formalism to deal with singular solutions. We show that the Lax pair formalism can be extended to a distributional weak Lax pair which is appropriate for dealing with the peakon solutions of the Camassa-Holm equation.

This dissertation deals with a class of nonlinear wave equations of the type discovered by R. Camassa and D. D. Holm which includes the Camassa-Holm, the Degasperis-Procesi, and the two component Camassa-Holm equations. All these equations admit certain non-smooth soliton-like solutions, called peakons as well as other non-smooth solutions like cuspons. We apply the techniques of the theory of distributions of L. Schwartz to study these solutions. In particular, every non-smooth traveling wave which is a distributional solution of the two component Camassa-Holm equation is a distributional solution of the Camassa-Holm equation if the set of points where the height of the wave equals its speed, is of measure zero. This includes peakon or cuspon traveling wave solutions.<p>We also develop a suitable modification of the classical Lax pair formalism to deal with singular solutions. We show that the Lax pair formalism can be extended to a distributional weak Lax pair which is appropriate for dealing with the peakon solutions of the Camassa-Holm equation.

A non-supersymmetric Yi-deformed AdS/CFT correspondence has recently been conjectured by Frolov. A detailed description of both sides of this proposed gauge/string duality is presented. The analogy that exists between single trace gauge theory operators in the SU(3) sector and i-deformed SU(3) integrable spin chains is also discussed. Frolov, Roiban and Tseytlin’s leading order comparison between the ideformed spin chain coherent state action and i-deformed string worldsheet action in the semiclassical limit is reviewed. A particular Lax pair representation for the first order semiclassical i-deformed spin chain/string action is then constructed.

Short Pulse equation derived by Schafer and Wayne is a nonlinear partial differential equation that describes ultra short laser propagation in a dispersive optical medium such as optical fibers. Some properties of this equation e.g. traveling wave solution and its soliton structure and some of its conserved quantities were investigated. Conserved quantities were obtained by mass conservation law, lax pair method and transformation between Sine-Gordon and short pulse equation. As a result, loop soliton characteristic and six conserved quantities were found.

In this thesis, we investigate the integrability properties of some evolutionary type nonlinear equations in (1+1)-dimensions both with commutative and non-commutative variables. We construct the recursion operators, based on the Lax representation, for such equations. Finally, we question the notion of integrability for a certain one-component non-commutative equation. [We stress that calculations in this thesis are not original.]

Στην παρούσα εργασία παρουσιάζεται μία μέθοδος πλήρους διακριτοποίησης (χωρικής και χρονικής) για την εξίσωση των Korteweg και de Vries. H μέθοδος αυτή μελετήθηκε από τον J. Schiff στην εργασία Loop groups and discrete KdV equations και στηρίζεται στην διάσπαση Birkhoff σε κατάλληλη ομάδα βρόχων για την εύρεση του ζεύγους Lax. Για τις προκύπτουσες εξισώσεις μερικών διαφορών κατασκευάζονται μετασχηματισμοί Backlund μέσω της ίδιας μεθόδου, οι οποίοι, στην συνέχεια, χρησιμοποιούνται για την εύρεση σολιτονικών λύσεων. Ειδικότερα, μία από τις διακριτοποιήσεις έχει άμεσο ("φυσικό") συνεχές όριο την εξίσωση potential KdV. Σε κάθε περίπτωση διακριτοποίησης, κατασκευάζονται σολιτονικές λύσεις, οι οποίες συγκρίνονται με αυτές της συνεχούς περίπτωσης και εξετάζονται ως προς την σολιτονική αλληλεπίδραση. / In this thesis, we present a method of full discretization (both spatial and temporal coordinates are discretized) for the Korteweg and de Vries' equation. This method was studied by J. Schiff in his paper Loop groups and discrete KdV equations. The procedure is based on Birkhoff decomposition in an appropriate loop group in order to derive a Lax representation. For the resulting partial difference equations, we construct Backlund transformations via the same method, which are used to generate soliton solutions. In particular, one discretization has the potential KdV equation as a standard (natural) continuum limit. In both cases, soliton solutions are produced and compared with those of the continuous case. Finally, we study their soliton interaction.

We present Lax pairs and functions called Lax functions corresponding to Calogero- Moser-Sutherland (CMS) systems. We present the Benjamin-Ono (BO) equation and a pole ansatz to the BO equation, constructed from a specific type of Lax function called a special Lax function corresponding to Rational and Trigonometric CMS systems. We present a generalization of the BO equation called the non-chiral Intermediate wave (ncILW) equation and show that a family of solutions to the ncILW equation can be constructed from the special Lax function corresponding to the hyperbolic CMS system. We present the Szegö equation on the circle and the real line. We obtain a family of solutions to the Szegö equation on the real line using a pole ansatz. Using numerical methods, we display solution plots to the BO equation and Szegö equation.

On étudie dans cette thèse l'équation de Szegö sur la droite réelle ainsi que ses perturbations. Cette équation a été introduite il y a quelques années par Gérard et Grellier comme modèle mathématique d'une équation non linéaire totalement non dispersive.L'équation de Szegöapparait naturellement dans l'étude de l'équation de Schrödinger non linéaire (NLS) danscertaines situations sur-critiques où l'on constate un manque de dispersion, par exemplelorsque l'on considère NLS sur le groupe de Heisenberg. Par conséquent, une des motivationsde cette thèse est d'établir des résultats concernant l'équation de Szegö qui pourrontéventuellement être utilisés dans le contexte de l'équation de Schrödinger non linéaire.Le premier résultat de cette thèse est la classification des solitons de l'équation de Szegö.On montre que ce sont tous des fonctions rationnelles ayant un unique pôle qui est simple.De plus, on prouve que les solitons sont orbitalement stables.La propriété la plus remarquable de l'équation de Szegö est le fait qu'elle est complètement intégrable, ce qui permet notamment d'établir une formule explicite de sa solution.Comme applications de cette formule, on obtient les trois résultats suivants. (A) On montreque les solutions fonctions rationnelles génériques se décomposent en une somme de solitonset d'un reste qui est petit lorsque le temps tend vers l'infini. (B) On met en évidence unexemple de solution non générique dont les grandes normes de Sobolev tendent vers l'infiniavec le temps. (C) On détermine des coordonnées action-angle généralisées lorsque l'on restreintl'équation de Szegö à une sous-variété de dimension finie. En particulier, on en déduitqu'une grande partie des trajectoires de cette équation sont des spirales autour de cylindrestoroïdaux.Comme l'équation de Szegö est complètement intégrable, il est ensuite naturel d'étudierses perturbations et d'établir de nouvelles propriétés pour celles-ci à partir des résultatsconnus pour l'équation de Szegö. Une des perturbations de l'équation de Szegö est une équation desondes non linéaire (NLW) de donnée bien préparée.On prouve que si la donnée initiale de NLW est petite et à support dans l'ensemble desfréquences positives, la solution de NLW est alors approximée pour un temps long par lasolution de l'équation de Szegö. Autrement dit, on démontre ainsi que l'équation de Szegöest la première approximation de NLW. On construit ensuite une solution de NLW dont lesgrandes normes de Sobolev augmentent (relativement à la norme de la donnée initiale).Sur le tore T, Gérard et Grellier ont démontré un résultat analogue d'approximation deNLW. On améliore ce résultat en trouvant une approximation plus fine, de deuxième ordre.Dans une dernière partie, on s'intéresse à l'équation de Szegö perturbée par un potentielmultiplicatif petit. On étudie l'interaction de ce potentiel avec les solitons. Plus précisément,on montre que, si la donnée initiale est celle d'un soliton pour l'équation non perturbée, lasolution de l'équation perturbée garde la forme d'un soliton sur un long temps. De plus, ondéduit la dynamique effective, i.e. les équations différentielles satisfaites par les paramètresdu soliton. / In this Ph.D. thesis, we study the Szegö equation on the real lineas well as its perturbations.It was recently introduced by Gérard and Grellier as a toy model of a non-lineartotally non dispersive equation. The Szegö equation appears naturally in the study of thenon-linear Schrödinger equation (NLS) in super-critical situations where dispersion lacks,for example, when one considers NLS on the Heisenberg group. Consequently, one of themotivations of this Ph.D. thesis is fi nding new results for the Szegö equation in hope thatthey could be eventually used in the context of the non-linear Schrödinger equation.Our first result is a classification of the solitons of the Szegö equation. We show thatthey are all rational functions with one simple pole. In addition, we prove the orbitalstability of solitons.The Szegö equation has the remarkable property of being completely integrable. Thisallows us to find an explicit formula for solutions. We obtain three applications of thisformula. (A) We prove soliton resolution for solutions which are generic rational functions.(B) We construct an example of non-generic solution whose high Sobolev norms grow toinfinity over time. (C) We find generalized action-angle variables when restricting the Szegöequation to a finite dimensional sub-manifold. In particular, this yields that most of thetrajectories of the Szegö equation are spirals around toroidal cylinders.Since the Szegö equation is completely integrable, it is natural to study its perturbationsand deduce new properties of such perturbations from the known results for the Szegöequation. One perturbation of the Szegö equation is a non-linear wave equation(NLW) with small initial data.We prove that the Szegö equation is the first order approximation of NLW. More precisely,if an initial condition of NLW is small and supported only on non-negative frequencies, thenthe corresponding solution can be approximated by the solution of the Szegö equation, fora long time. We then construct a solution of NLW whose high Sobolev norms grow.On the torus T, Gérard and Grellier proved an analogous first order approximationresult for NLW. By considerning the second order approximation, we obtain an improvedresult with a smaller error.Lastly, we consider the Szegö equation perturbed by a small multiplicative potential.We study the interaction of this potential with solitons. More precisely, we show that, if theinitial condition is that of a soliton for the unperturbed Szegö equation, then the solutionpreserves the shape of a soliton for a long time. In addition, we prescribe the effectivedynamics, i.e. we derive the differential equations satisfied by the parameters of the soliton.