Peakon integrable nonlinear equations and related approximation problems: the distributional approach.

点击翻译按钮获取中文摘要

Integrable partial differential equations arose as approximate model equations in the mathematical description of wave phenomena in fluids, plasmas and optical fibres. They are special in that they possess infinitely many integrals of motion, a sign of a rich mathematical structure. P. Lax pointed out at the very early stage of research into this special class of partial differential equations that the underlying reason for the existence of infinitely many conserved quantities is that one can associate with each of these equations a linear operator which undergoes an isospectral (spectrum preserving) deformation responsible for the existence of a large family of constants of motion. This point of view eventually resulted in the formulation of a paradigm of integrable systems, which puts the so called Lax pairs front and centre of the theory. This point of view has its geometric counterpart in a theory of flat connections, as well as, in a symplectic approach based on the presence of bi-Hamiltonian structures. ******The current proposal focuses on the first interpretation, using Lax pairs and their isospectral deformations, with one significant generalization: the Lax pairs are distributional (Schwartz) Lax pairs. The main impetus for this line of research comes from a class of exciting integrable models discovered over last two decades which exhibit several novel features, the central of which is the existence of localized coherent modes, called peakons, with singular behaviour in the spatial derivative of the profile. The peakon equations result from a careful consideration of distributional Lax pairs leading to interesting and unexpected results, i.e. forcing a particular type of multiplication rules for certain classes of distributions with singular support, leading to a relevant extension of “Lax integrable” equations into the domain of significant non-smoothness for which the classical Lax formalism fails. ******The general objectives of the proposed program are: ***(1) to establish a complete theory of isospectral deformations of the n-th order inhomogeneous strings (n=2, n=3, n=4, corresponding to a classical string, a cubic string, an Euler-Bernoulli beam, respectively) and the role of, possibly generalized, Cauchy biorthogonal polynomials for n greater than 3. This, in particular, would address the inverse problem for the Euler-Bernoulli beam which is of significance elsewhere in view of its relevance to the geophysical inverse problem; ***(2) to classify peakon distributional Lax pairs and pertinent boundary value problems; ***(3) to build a theory of mixed Hermite-Pade approximations of the so called Nikishin systems, generalizing earlier results on the Nikishin systems with two measures originating in the cubic string problem formulated by the applicant for the Degasperis-Procesi equation and establish a link between these approximations and inverse problems for n-order inhomogeneous strings. ********