Math @ Duke

Publications [#244165] of Stephanos Venakides
Papers Published
 Tovbis, A; Venakides, S, Nonlinear steepest descent asymptotics for semiclassical limit of Integrable systems: Continuation in the parameter space,
Communications in Mathematical Physics, vol. 295 no. 1
(2010),
pp. 139160, ISSN 00103616 [doi]
(last updated on 2018/06/18)
Abstract: The initial value problem for an integrable system, such as the Nonlinear Schrödinger equation, is solved by subjecting the linear eigenvalue problem arising from its Lax pair to inverse scattering, and, thus, transforming it to a matrix RiemannHilbert problem (RHP) in the spectral variable. In the semiclassical limit, the method of nonlinear steepest descent ([4,5]), supplemented by the gfunction mechanism ([3]), is applied to this RHP to produce explicit asymptotic solution formulae for the integrable system. These formule are based on a hyperelliptic Riemann surface R = R(x, t) in the spectral variable, where the spacetime variables (x, t) play the role of external parameters. The curves in the x, t plane, separating regions of different genuses of R(x, t), are called breaking curves or nonlinear caustics. The genus of R(x, t) is related to the number of oscillatory phases in the asymptotic solution of the integrable system at the point x, t. The evolution theorem ([10]) guarantees continuous evolution of the asymptotic solution in the spacetime away from the breaking curves. In the case of the analytic scattering data f(z; x, t) (in the NLS case, f is a normalized logarithm of the reflection coefficient with time evolution included), the primary role in the breaking mechanism is played by a phase function h(z; x, t), which is closely related to the g function. Namely, a break can be caused ([10]) either through the change of topology of zero level curves of h(z; x, t) (regular break), or through the interaction of zero level curves of h(z; x, t) with singularities of f (singular break). Every time a breaking curve in the x, t plane is reached, one has to prove the validity of the nonlinear steepest descent asymptotics in the region across the curve. In this paper we prove that in the case of a regular break, the nonlinear steepest descent asymptotics can be "automatically" continued through the breaking curve (however, the expressions for the asymptotic solution will be different on the different sides of the curve). Our proof is based on the determinantal formula for h(z; x, t) and its space and time derivatives, obtained in [8,9]. Although the results are stated and proven for the focusing NLS equation, it is clear ([9]) that they can be reformulated for AKNS systems, as well as for the nonlinear steepest descend method in a more general setting. © SpringerVerlag 2010.


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