6533b825fe1ef96bd1281bec

RESEARCH PRODUCT

On the semiclassical limit of the defocusing Davey-Stewartson II equation

subject

description

Inverse scattering is the most powerful tool in theory of integrable systems. Starting in the late sixties resounding great progress was made in (1+1) dimensional problems with many break-through results as on soliton interactions. Naturally the attention in recent years turns towards higher dimensional problems as the Davey-Stewartson equations, an integrable generalisation of the (1+1)-dimensionalcubic nonlinear Schrödinger equation. The defocusing Davey-Stewartson II equation, in its semi-classical limit has been shown in numerical experiments to exhibit behavior that qualitatively resembles that of its one-dimensional reduction, namely the generation of a dispersive shock wave: smooth initial data develop a zone rapid modulated oscillations in the vicinity of shocks of solutions for the corresponding dispersionless equations for the same initial data. The present thesis provides a first step to study this problem analytically using the inverse scattering transform method. Both the direct and inverse scattering transform for DSII can be expressed as D-bar equations. We consider the direct spectral transform for the defocusing Davey-Stewartson II equation for smooth initial data in the semi-classical limit. The direct spectral transform involves a singularly perturbed elliptic Dirac system in two dimensions. We introduce a WKB-type method for this problem and prove that it is well defined for sufficiently large modulus of the spectral parameter k ∈ ℂ by controlling the solution of an associated nonlinear eikonal problem. Further, we give numerical evidence that the method is accurate for such k in the semiclassical limit. Producing this evidence requires both the numerical solution of the singularly perturbed Dirac system and the numerical solution of the eikonal problem. We present a new method for the numerical solution of the eikonal problem valid for sufficiently large |k|. For a particular potential we are able to solve the eikonal problem in a closed form for all k, acalculation that yields some insight into the failure of the WKB method for smaller values of |k|. The numerical calculations of the direct spectral transform indicate how to study the singularly perturbed Dirac system for values of |k| so small that there is no global solution of the eikonal problem. We provide a rigorous semiclassical analysis of the solution for real radial potentials at k=0, which yields an asymptotic formula for the reflection coefficient at k = 0 and suggests an annular structure for the solution that may be exploited when |k| ≠ 0 is small. The numerics also suggest that for some potentials the reflection coefficient converges point-wise as ε ↓ 0 to a limiting function that is supported in the domain of k-values on which the eikonal problem does not have a global solution. We suggest that singularities of the eikonal function play a role similar to that of turning points in the one-dimensional theory.