A scalar Riemann-Hilbert problem on the torus: Applications to the KdV equation

We take a closer look at the Riemann-Hilbert problem associated to one-gap solutions of the Korteweg-de Vries equation. To gain more insight, we reformulate it as a scalar Riemann-Hilbert problem on the torus. This enables us to derive deductively the model vector-valued and singular matrix-valued solutions in terms of Jacobi theta functions. We compare our results with those obtained in recent literature.

Riemann-Hilbert Theory without local Parametrix Problems: Applications to Orthogonal Polynomials

We study whether in the setting of the Deift-Zhou nonlinear steepest descent method one can avoid solving local parametrix problems explicitly, while still obtaining asymptotic results. We show that this can be done, provided an a priori estimate for the exact solution of the Riemann-Hilbert problem is known. This enables us to derive asymptotic results for orthogonal polynomials on $[-1,1]$ with a new class of weight functions. In these cases, the weight functions are too badly behaved to allow a reformulation of a local parametrix problem to a global one with constant jump matrices. Possible implications for edge universality in random matrix theory are also discussed.

Parametrix problem for the Korteweg--de Vries equation with steplike initial data

In this paper we study the asymptotics of the Korteweg--de Vries (KdV) equation with steplike initial data, which leads to shock waves, in the middle region between the dispersive tail and the soliton region, as $t rightarrow infty$. In our previous work we have dealt with this question, but failed to obtain uniform estimates in $x$ and $t$ because of the previously unknown singular behaviour of the matrix model solution. The main goal of this paper is to close this gap. We present an alternative approach to the usual argument involving a small norm Riemann--Hilbert (R-H) problem, which is based instead on Fredholm index theory for singular integral operators. In particular, we avoid the construction of a global model matrix solution, which would be singular for arbitrary large $x$ and $t$, and utilize only the symmetric model vector solution, which always exists and is unique.

On vector and matrix Riemann-Hilbert problems for KdV shock waves

This paper discusses some general aspects and techniques associated with the long-time asymptotics of steplike solutions of the Korteweg-de Vries (KdV) equation via vector Riemann--Hilbert problems. We also elaborate on an ill-posedness of the matrix Riemann-Hilbert problems for the KdV case. To the best of our knowledge this is the first time such ill-posedness is discussed in applications of Riemann--Hilbert theory. Furthermore, we rigorously justify the asymptotics for the shock wave in the elliptic zone derived previously.