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## PDE/Analysis seminar: Bing-Ying Lu "Universality near the gradient catastrophe in the semi-classical sine-Gordon equation"

### Description

Speaker: Bing-Ying Lu, Institute of Mathematics, Taipei, Taiwan

Title: Universality near the gradient catastrophe in the semi-classical sine-Gordon equation

Abstract:
We study the semiclassical limit of the sine-Gordon (sG) equation with below threshold pure impulse initial data of Klaus--Shaw type. The Whitham averaged approximation of this system exhibits a gradient catastrophe in finite time. In accordance with a conjecture of Dubrovin, Grava and Klein, we found that in a $\mathcal{O}(\epsilon^{4/5})$ neighborhood near the gradient catastrophe point, the asymptotics of the sG solution are universally described by the Painlev\'e I tritronqu\'ee solution. A linear map can be explicitly made from the tritronqu\'ee solution to this neighborhood. Under this map: away from the tritronqu\'ee poles, the first correction of sG is universally given by the real part of the Hamiltonian of the tritronqu\'ee solution; localized defects appear at locations mapped from the poles of tritronqu\'ee solution; the defects are proved universally to be a two parameter family of special localized solutions on a periodic background for the sG equation. We are able to characterize the solution in detail. Our approach is the rigorous steepest descent method for matrix Riemann--Hilbert problems, substantially generalizing Bertola and Tovbis's results on the nonlinear Schrödinger equation to establish universality beyond the context of solutions of a single equation.
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