Long time and Painlevé-type asymptotics for the defocusing Hirota equation with finite density initial data

Abstract

In this work, we consider the Cauchy problem for the defocusing Hirota equation with a nonzero background \begin{align} \begin{cases} iq_{t}+α\left[q_{xx}-2\left(\left\vert q\right\vert^{2}-1\right)q\right]+iβ\left(q_{xxx}-6\left\vert q\right\vert^{2}q_{x}\right)=0,\quad (x,t)\in \mathbb{R}\times(0,+\infty),\\ q(x,0)=q_{0}(x),\qquad \underset{x\rightarrow\pm\infty 1}{\lim} q_{0}(x)=\pm 1, \qquad q_{0}\mp 1\in H^{4,4}(\mathbb{R}). \end{cases} \nonumber \end{align} According to the Riemann-Hilbert problem representation of the Cauchy problem and the $\bar{\partial }$ generalization of the nonlinear steepest descent method, we find different long time asymptotics types for the defocusing Hirota equation in oscillating region and transition region, respectively. For the oscillating region $\xi <-8$, four phase points appear on the jump contour $\mathbb{R}$, which arrives at an asymptotic expansion,given by \begin{align} q(x,t)=-1+t^{-1/2}h+O(t^{-3/4}).\nonumber \end{align} It consists of three terms. The first term $-1$ is leading term representing a nonzero background, the second term $t^{-1/2}h$ originates from the continuous spectrum and the third term $O(t^{-3/4})$ is the error term due to pure $\bar{\partial }$-RH problem. For the transition region $|\xi +8|t^{2/3}<C$, three phase points raise on the jump contour $\mathbb{R}$. Painlevé asymptotics expansion is obtained \begin{align} q(x,t)=-1-(\frac{15}{4}t)^{-1/3}\varrho+O(t^{-1/2}),\nonumber \end{align} in which the leading term is a solution to the Painlevé II equation, the last term is a residual error being from pure $\bar{\partial }$-RH problem and parabolic cylinder model.