Scattering for the cubic Klein--Gordon equation in two space dimensions

TL;DR

This paper proves global existence and spacetime bounds for the cubic Klein--Gordon equation in two dimensions, linking its behavior to known results for the nonlinear Schr"odinger equation, and characterizes blowup conditions.

Contribution

It establishes new global and blowup criteria for the 2D cubic Klein--Gordon equation based on Schr"odinger equation analogues, extending previous results.

Findings

Solutions are global in the defocusing case with finite spacetime bounds.

In the focusing case, a dichotomy between global existence and blowup is characterized.

Results are mostly unconditional, relying on known Schr"odinger equation results.

Abstract

We consider both the defocusing and focusing cubic nonlinear Klein--Gordon equations $$ u_{tt} - \Delta u + u \pm u^3 =0 $$ in two space dimensions for real-valued initial data $u(0)\in H^1_x$ and $u_t(0)\in L^2_x$. We show that in the defocusing case, solutions are global and have finite global $L^4_{t,x}$ spacetime bounds. In the focusing case, we characterize the dichotomy between this behaviour and blowup for initial data with energy less than that of the ground state. These results rely on analogous statements for the two-dimensional cubic nonlinear Schr\"odinger equation, which are known in the defocusing case and for spherically-symmetric initial data in the focusing case. Thus, our results are mostly unconditional. It was previously shown by Nakanishi that spacetime bounds for Klein--Gordon equations imply the same for nonlinear Schr\"odinger equations.