A quadratic quasi-linear Klein–Gordon equation in two space dimensions

We consider the quasi-linear Klein–Gordon equations in two space dimensions \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\left(\partial_{t}^{2} - \Delta + 1\right) u=\mathcal{N} (u)$$\end{document}in \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${(t, x) \in \mathbf{R} \times \mathbf{R}^{2}}$$\end{document} with a quadratic nonlinearity \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathcal{N} (u)}$$\end{document}, which is linear with respect to the second-order derivatives of unknown functions.