Almost global solutions of 2-D quadratic quasilinear wave equations in exterior domains without the second null condition

Abstract

For 3-D quadratic quasilinear wave equations in exterior domains with homogeneous Dirichlet boundary conditions and small initial data of size $\varepsilon>0$, the authors of two papers [M. Keel, H. Smith, C.D. Sogge, Amer. J. Math. 122 (2000), no. 4, 805-842; J. Amer. Math. Soc. 17 (2004), no. 1, 109-153] have shown that the lifespan ${\bar T}_{\varepsilon}$ of smooth solutions satisfies ${\bar T}_{\varepsilon}=\infty$ or ${\bar T}_{\varepsilon}\ge e^{C/\varepsilon}$, depending on whether the null condition holds or not, respectively. However, for the corresponding 2-D problem in exterior domains, it remains a difficult and unsolved problem to determine the precise lifespan $T_{\varepsilon}$ of smooth solutions. In our previous papers, we have established that $T_{\varepsilon}=\infty$ holds when both the first and second null conditions are satisfied; and that $T_{\varepsilon}\ge\frac{C}{\varepsilon^2}$ holds when the first null condition is violated. In this paper, we will prove $T_{\varepsilon}\ge e^{C/\varepsilon^2}$ when the first null condition holds but the second null condition is violated. Combining these results, we have systematically established sharp estimates for the lifespan $T_{\varepsilon}$ of 2-D quadratic quasilinear wave equations in exterior domains with homogeneous Dirichlet boundary conditions. Moreover, our series of results for quasilinear wave equations in 2-D exterior domains are consistent with the corresponding results for the classical Cauchy problem.