A Note on Lower Bound Lifespan Estimates for Semi-linear Wave/Klein–Gordon Equations Associated with the Harmonic Oscillator

In this paper, we show that for almost every m>0\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$m>0$$\end{document}, the solution to the semi-linear Klein–Gordon equation associated with the harmonic oscillator, with small initial data, exists over a longer time interval than the one given by local existence theory, using the normal form method. A similar result for the quadratic wave equation is also obtained.