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This thesis is focusing on boundary problems for various classical integrable schemes. First, we consider the vector nonlinear Schrodinger (NLS) equation on the halfline. Using a Backlund transformation method which explores the folding symmetry of the system, classes of integrable boundary conditions (BCs) are derived. These BCs coincide with the linearizable BCs obtained using the unified transform method developed by Fokas. The notion of integrability is argued by constructing an explicit generating function for conserved quantities. Then, by adapting a mirror image technique, an inverse scattering method with an integrable boundary is constructed in order to obtain N-soliton solutions on the half-line, i.e. N-soliton reflections. An interesting phenomenon of transmission between different components of vector solitons before and after interacting with the boundary is demonstrated. Next, in light of the fact that the soliton-soliton interactions give rise to Yang-Baxter maps, we realize that the soliton-boundary interactions that are extracted from the N-soliton reflections can be translated into maps satisfying the set-theoretical counterpart of the quantum reflection equation. Solutions of the set-theoretical reflection equation are referred to as reflection maps. Both the Yang-Baxter maps and the reflection maps guarantee the factorization of the soliton-soliton and soliton-boundary interactions for vector NLS solitons on the half-line. Indeed, reflection maps represent a novel mathematical structure. Basic notions such as parametric reflection maps, their graphic representations and transfer maps are also introduced. As a natural extension, this object is studied in the context of quadrirational Yang-Baxter maps, and a classification of quadrirational reflection maps is obtained. Finally, boundaries are added to discrete integrable systems on quad-graphs. Triangle configurations are used to discretize quad-graphs with boundaries. Relations involving vertices of the triangles give rise to boundary equations that are used to described BCs. We introduce the notion of integrable BCs by giving a three-dimensional boundary consistency as a criterion for integrability. By exploring the correspondence between the quadrirational Yang-Baxter maps and the so-called ABS classification, we also show that quadrirational reflection maps can be used as a systematic tool to generate integrable boundary equations for the equations from the ABS classification.

In chapter two, we establish new results for periodic solutions of some second order non-linear boundary value problems. We develop the upper and lower solutions method to show existence of solutions in the closed set defined by the well ordered lower and upper solutions. We develop the method of quasilinearization to approximate our problem by a sequence of solutions of linear problems that converges to the solution of the original problem quadratically. Finally, to show the applicability of our technique, we apply the theoretical results to a medical problem namely, a biomathematical model of blood flow in an intracranial aneurysm. In chapter three we study some nonlinear boundary value problems with nonlinear nonlocal three-point boundary conditions. We develop the method of upper and lower solutions to establish existence results. We show that our results hold for a wide range of nonlinear problems. We develop the method of quasilinearization and show that there exist monotone sequences of solutions of linear problems that converges to the unique solution of the nonlinear problems. We show that the sequences converge quadratically to the solutions of the problem in the C1 norm. We generalize the technique by introducing an auxiliary function to allow weaker hypotheses on the nonlinearity involved in the differential equations. In chapter four, we extend the results of chapter three to nonlinear problems with linear four point boundary conditions. We generalize previously existence results studied with constant lower and upper solutions. We show by an example that our results are more general. We develop the method of quasilinearization and its generalization for the four point problems which to the best of our knowledge is the first time the method has been applied to such problems. In chapter five, we extend the results to second order problems with nonlinear integral boundary conditions in two separate cases. In the first case we study the upper and lower solutions method and the generalized method of quasilinearization for the Integral boundary value problem with the nonlinearity independent of the derivative. While in the second case we show the nonlinearity to depend also on the first derivative. Finally, in chapter six, we study multiplicity results for three point nonlinear boundary value problems. We use the method of upper and lower solutions and degree arguments to show the existence of at least two solutions for certain range of a parameter r and no solution for other range of the parameter. We show by an example that our results are more general than the results studied previously. We also study existence of at least three solutions in the pressure of two lower and two upper solutions for some three-point boundary value problems. In one problem, we employ a condition weaker than the well known Nagumo condition which allows the nonlinearity f(t, x, x’) to grow faster than quadratically with respect to x’ in some cases.

Typescript / Bibliography: leaves 145-150 / vii, 150 leaves : ill ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--University of Adelaide, Dept. of Applied Mathematics, 1988

Η παρούσα διατριβή εκπονήθηκε ως μια διπλώματική εργασία υπό την επίβλεψη του καθηγητή Αναστάσιου Μπούντη (Τμήμα Μαθηματικό Πανεπιστήμιο Πατρών), κατά την διάρκεια του ακαδημαικού έτους 2012-2013. Στόχος μας ήταν να μελετήσουμε τόσο θεωρητικά όσο και αριθμητικά μη τετριμένες λύσεις και να κατανοήσουμε, σε γενικές γραμμές, τη συμπεριφορά της μη γραμμικής διαφορικής εξίσωσης δεύτερης τάξης van der Pol. Στη μελέτη που ακολουθεί εξετάζονται δύο περιπτώσεις της εξίσωσης van der Pol: η αυτόνομη μορφή και η μή αυτόνομη με περιοδικό εξαναγκασμό. Η εξίσωση που μελετάμε είναι μη γραμμική, οπότε για την ανάλυσή της χρησιμοποιείται η θεωρία διαταραχών μη γραμμικών διαφορικών εξισώσεων. Η θεωρία αυτή χρησιμοποιείται για τη κατασκευή προσεγγιστικών λύσεων, οι οποίες στη συνέχεια συγκρίνονται με τα αντίστοιχα αποτελέσματα που παράγονται μέσω αριθμητικής ολοκλήρωσης. Σχολιάζονται οι ομοιότητες και οι διαφορές μεταξύ των μεθόδων, τα πλεονεκτήματα και οι αδυναμίες τους. Συζητούνται επίσης ορισμένες από τις πιο χαρακτηριστικές ιδιότητες των λύσεων τόσο στη αυτόνομη, όσο και τη μη αυτόνομη μορφή της εξίσωσης. Ειδικότερα στο Kεφάλαιο 2 επικεντρωνόμαστε στην αυτόνομη μορφή και παραθέτουμε βασικούς ορισμούς και θεωρήματα της θεωρίας μη γραμμικών Σ.Δ.Ε, για την ποιοτική μελέτη της εξίσωσης. Μελετάται το είδος και η ευστάθεια των σημείων ισορροπίας και αποδεικνύεται η ύπαρξη οριακού κύκλου μέσω της θεωρίας Poincare-Bendixson. Με χρήση των μεθόδων ασυμπτωτικής επέκτασης, Poincare-Lindstedt και πολλαπλών χρονικών κλιμάκων της θεωρίας διαταραχών, προσδιορίζονται διαφορετικές προσεγγίσεις του οριακού κύκλου της εξίσωσης για 0<ε<<1. Σε κάθε περίπτωση κατασκευάζονται συγκριτικά διαγράμματα, όπου περιγράφονται οι λύσεις που δίνουν η αριθμητική ολοκλήρωση και οι αναλυτικές προσεγγίσεις. Στο Kεφάλαιο 3 αναλύονται μη αυτόνομες μορφές της εξίσωσης και διακρίνονται δύο περιπτώσεις: Διέγερση συχνότητας κοντά σε αυτή του αυτόνομου συστήματος και διέγερση συχνότητας μακριά από αυτή του αυτόνομου συστήματος. Στην πρώτη περίπτωση υπολογίζονται προσεγγίσεις των περιοδικών λύσεων της εξίσωσης με τις μεθόδους Poincare-Lindstedt και πολλαπλών χρονικών κλιμάκων και παρουσιάζονται σε διαγράμματα οι περιοδικές και οι σχεδόν-περιοδικές λύσεις για ορισμένες τιμές των παραμέτρων. Στη δεύτερη περίπτωση υπολογίζονται προσεγγιστικές λύσεις με τη μέθοδο δύο χρονικών κλιμάκων και κατασκευάζονται συγκριτικά διαγράμματα με τη λύση που δίνει η αριθμητική ολοκλήρωση, για τιμές παραμέτρων που αντιστοιχούν σε περιοδικές και σχεδόν-περιοδικές καταστάσεις. Στο τέλος του κεφαλαίου δείχνεται η ύπαρξη χαοτικής συμπεριφοράς στο σύστημα μας. Το Παράρτημα Α περιλαμβάνει τα κυριότερα στοιχεία, ορισμούς και θεωρήματα, της θεωρίας μη γραμμικών Σ.Δ.Ε, τα οποία αναφέρονται και εφαρμόζονται στα Κεφάλαια 2 και 3. Τέλος περέχονται όλα τα προγράμματα σε Mathematica, με τα οποία κατασκευάστηκαν τα διαγράμματα της εργασίας και πραγματοποιήθηκε η αριθμητική ολοκλήρωση των εξισώσεων. / This thesis elaborated as diploma work under the supervision of Professor Anastasios Buddhi (Department of Mathematics University of Patras), during the academic year 2012-2013. Our aim was to study both theoretically and numerically non- trivial solutions and to understand, in general, the behavior of non- linear differential equation of second order van der Pol. The following study examined both cases the equation van der Pol: the independent form and the non- autonomous by periodic forcing. The equation is nonlinear study, so for analysis using the perturbation theory of nonlinear differential equations. The theory is used to construction of approximate solutions which then compared with the corresponding results obtained through numerical integration. Commented on the similarities and differences between the methods, strengths and weaknesses. Also discussed some of the most characteristic properties of the solutions both autonomous and non- autonomous form of the equation. In particular in Chapter 2 we focus on autonomous form and quote basic definitions and theorems of the theory of nonlinear SDE for the qualitative study of the equation. studied the nature and stability of equilibria and prove the existence of incremental cycle through theory Poincare-Bendixson. Using the methods asymptotic expansion Poincare-Lindstedt and multiple time scales of perturbation theory, identified different approaches boundary circle of the equation for 0 < e << 1. In each case made comparative charts describing the solutions that give the numerical integration and analytical approaches. In Chapter 3 details the forms of non- autonomous equation there are two cases: Excitation frequency close to that of autonomous system and stimulation frequency from that of autonomous system. In the first case calculated approximations of periodic solutions of the equation by the methods of Poincare-Lindstedt and multiple time scales and charts presented in periodic and quasi- periodic solutions for certain values ​​of parameters. In the second case calculated approximate solutions with method two time scales and made ​​comparatively charts with the solution gives the numerical integration for parameter values ​​corresponding to periodic and quasi- periodic statements. At the end of the chapter shows the existence of chaotic behavior in our system.  Appendix A contains the main elements and definitions theorems of the theory of nonlinear SDE, which referred to and applied in Chapters 2 and 3. Finally presentation given all programs in Mathematica, the constructed diagrams of work and performed the numerical integration of the equations.