Geometric Analysis of Nonlinear Partial Differential Equations

This book contains a collection of twelve papers that reflect the state of the art of nonlinear differential equations in modern geometrical theory. It comprises miscellaneous topics of the local and nonlocal geometry of differential equations and the applications of the corresponding methods in hydrodynamics, symplectic geometry, optimal investment theory, etc. The contents will be useful for all the readers whose professional interests are related to nonlinear PDEs and differential geometry, both in theoretical and applied aspects

Nonlinear classical and quantum integrable systems with PT -symmetries

A key feature of integrable systems is that they can be solved to obtain exact analytical solutions. In this thesis we show how new models can be found through generalisations of some well known nonlinear partial differential equations including the Korteweg-de Vries, modified Korteweg-de Vries, sine-Gordon, Hirota, Heisenberg and Landau-Lifschitz types with joint parity and time symmetries whilst preserving integrability properties. The first joint parity and time symmetric generalizations we take are extensions to the complex and multicomplex fields, such as bicomplex, quaternionic, coquaternionic and octonionic types. Subsequently, we develop new methods from well-known ones, such as Hirota’s direct method, Bäcklund transformations and Darboux-Crum transformations to solve for these newsystems to obtain exact analytical solutions of soliton and multi-soliton types. Moreover, in agreement with the reality property present in joint parity and time symmetric non-Hermitian quantum systems, we find joint parity and time symmetries also play a key role for reality of conserved charges for the new systems, even though the soliton solutions are complex or multicomplex. Our complex extensions have proved to be successful in helping one to obtain regularized degenerate multi-soliton solutions for the Korteweg-de Vries equation, which has not been realised before. We extend our investigations to explore degenerate multi-soliton solutions for the sine-Gordon equation and Hirota equation. In particular, we find the usual time-delays from degenerate soliton solution scattering are time-dependent, unlike the non-degenerate multi-soliton solutions, and provide a universal formula to compute the exact time-delay values for scattering of N-soliton solutions. Other joint parity and time symmetric extensions of integrable systems we take are of nonlocal nature, with nonlocalities in space and/or in time, of time crystal type. Whilst developing new methods for the construction of soliton solutions for these systems, we xiv find new types of solutions with different parameter dependence and qualitative behaviour even in the one-soliton solution cases. We exploit gauge equivalence between the Hirota system with continuous Heisenberg and Landau-Lifschitz systems to see how nonlocality is inherited from one system to another and vice versa. In the final part of the thesis, we extend some of our investigations to the quantum regime. In particularwe generalize the scheme of Darboux transformations for fully timedependent non-Hermitian quantum systems, which allows us to create an infinite tower of solvable models

Symmetry Analysis, Exact Solutions and Conservation Laws of a Benjamin-Bona-Mahony-Burgers Equation in 2+1-Dimensions

The Benjamin-Bona-Mahony equation describes the unidirectional propagation of small-amplitude long waves on the surface of water in a channel. In this paper, we consider a family of generalized Benjamin-Bona-Mahony-Burgers equations depending on three arbitrary constants and an arbitrary function G(u). We study this family from the standpoint of the theory of symmetry reductions of partial differential equations. Firstly, we obtain the Lie point symmetries admitted by the considered family. Moreover, taking into account the admitted point symmetries, we perform symmetry reductions. In particular, for G & PRIME;(u)& NOTEQUAL;0, we construct an optimal system of one-dimensional subalgebras for each maximal Lie algebra and deduce the corresponding (1+1)-dimensional nonlinear third-order partial differential equations. Then, we apply Kudryashov's method to look for exact solutions of the nonlinear differential equation. We also determine line soliton solutions of the family of equations in a particular case. Lastly, through the multipliers method, we have constructed low-order conservation laws admitted by the family of equations.The authors acknowledge the financial support from Junta de Andalucia group FQM-201. The authors warmly thank the referees for their valuable comments and recommending changes that significantly improved this paper. In memory of Maria de los Santos Bruzon Gallego: thank you for dedicating your time and effort to care us and help us. You will always be our role model. May Maruchi rest in peace

Hydrodynamics

The phenomena related to the flow of fluids are generally complex, and difficult to quantify. New approaches - considering points of view still not explored - may introduce useful tools in the study of Hydrodynamics and the related transport phenomena. The details of the flows and the properties of the fluids must be considered on a very small scale perspective. Consequently, new concepts and tools are generated to better describe the fluids and their properties. This volume presents conclusions about advanced topics of calculated and observed flows. It contains eighteen chapters, organized in five sections: 1) Mathematical Models in Fluid Mechanics, 2) Biological Applications and Biohydrodynamics, 3) Detailed Experimental Analyses of Fluids and Flows, 4) Radiation-, Electro-, Magnetohydrodynamics, and Magnetorheology, 5) Special Topics on Simulations and Experimental Data. These chapters present new points of view about methods and tools used in Hydrodynamics

Research Advances in Chaos Theory

The subject of chaos has invaded practically every area of the natural sciences. Weather patterns are referred to as chaotic. There are chemical reactions and chaotic evolution of insect populations. Atomic and molecular physics have also seen the emergence of the study of chaos in these microscopic domains. This book examines the issue of chaos in nonlinear and dynamical systems, quantum mechanics, biology, and economics

Quantum backflow in the presence of defects

A physical quantity that is positive in classical physics can become negative in quantum physics, but it may be bounded. Quantum inequalities are lower bounds on averages of these physical quantities. In the case of energy densities of a quantum field, it is called a quantum energy inequality. In the case of the probability current density of right-moving states, it is called the quantum backflow effect. This thesis is concerned with various aspects of the quantum backflow effect in the presence of defects. The backflow effect states that a particle moving towards a reference point with positive momentum may have the probability of being found at the right of the reference point decreased with time. Defects represent a way of implementing generalised point interactions without necessarily having an explicit potential function to be added to the Hamiltonian of a physical system and are described by sewing conditions defined at the defect location. Starting from the Dirac delta-distribution, which can be regarded as a potential function but also as a point defect, we extend the analysis to the jump-defect, a discontinuous and purely transmitting integrable defect allowing conservation of total energy and momentum. In this thesis, we will examine how the backflow is affected in the presence of different defects giving special focus on the jump-defect, which does not have a backscattering contribution to the backflow constant and makes our analysis compatible with conservation laws. Beyond the Schrödinger equation, we will introduce and analyse backflow with defects in the Dirac equation, which takes into account the spin contribution to the probability current. The existence of bound states are shown to be relevant for the bounds on backflow, and numerical results will support that. Furthermore, we will investigate how the backflow constant in the presence of defects differs from the interaction-free situation