On Cauchy problem to the modified Camassa-Holm equation: Painlevé asymptotics

Abstract: We investigate the Painlev\'{e} asymptotics for the Cauchy problem of the modified Camassa-Holm (mCH) equation with zero boundary conditions \begin{align*}\nonumber &m_t+\left((u^2-u_x^2)m\right)_x=0, \ (x,t)\in\mathbb{R}\times\mathbb{R}^+,\ &u(x,0)=u_0(x), \lim_{x\to\pm\infty} u_0(x)=0, \end{align*} where $u_0(x)\in H^{4,2}(\mathbb{R})$. Recently, Yang and Fan (Adv. Math. 402, 108340 (2022)) reported the long-time asymptotic result for the mCH equation in the solitonic regions. The main purpose of our work is to study the long-time asymptotic behavior in two transition regions. The key to proving this result is to establish and analyze the Riemann-Hilbert problem on a new plane $(y;t)$ related to the Cauchy problem of the mCH equation. With the $\bar{\partial}$-generalization of the Deift-Zhou nonlinear steepest descent method and double scaling limit technique, in two transition regions defined by \begin{align}\nonumber \mathcal{P}{I}:={(x,t):0\leqslant |\frac{x}{t}-2|t^{2/3}\leqslant C},~~~~\mathcal{P}{II}:={(x,t):0\leqslant |\frac{x}{t}+1/4|t^{2/3}\leqslant C}, \end{align} where $C>0$ is a constant, we find that the leading order approximation to the solution of the mCH equation can be expressed in terms of the Painlev\'{e} II equation.