Electron. J. Differential Equations, Vol. 2016 (2016), No. 266, pp. 1-16.

Convergence of exterior solutions to radial Cauchy solutions for $\partial_t^2U-c^2\Delta U=0$

Helge Kristian Jenssen, Charis Tsikkou

Consider the Cauchy problem for the 3-D linear wave equation $\partial_t^2U-c^2\Delta U=0$ with radial initial data $U(0,x)=\Phi(x)=\varphi(|x|)$, $U_t(0,x)=\Psi(x)=\psi(|x|)$. A standard result states that U belongs to $C([0,T];H^s(\mathbb{R}^3))$ whenever $(\Phi,\Psi)\in H^s\times H^{s-1}(\mathbb{R}^3)$. In this article we are interested in the question of how U can be realized as a limit of solutions to initial-boundary value problems on the exterior of vanishing balls $B_\varepsilon$ about the origin. We note that, as the solutions we compare are defined on different domains, the answer is not an immediate consequence of $H^s$ well-posedness for the wave equation. We show how explicit solution formulae yield convergence and optimal regularity for the Cauchy solution via exterior solutions, when the latter are extended continuously as constants on $B_\varepsilon$ at each time. We establish that for s=2 the solution U can be realized as an $H^2$-limit (uniformly in time) of exterior solutions on $\mathbb{R}^3\setminus B_\varepsilon$ satisfying vanishing Neumann conditions along $|x|=\varepsilon$, as $\varepsilon\downarrow 0$. Similarly for s=1: U is then an $H^1$-limit of exterior solutions satisfying vanishing Dirichlet conditions along $|x|=\varepsilon$.

Submitted July 22, 2016. Published September 30, 2016.
Math Subject Classifications: 35L05, 35L15, 35L20.
Key Words: Linear wave equation; Cauchy problem; radial solutions; exterior solutions; Neumann and Dirichlet conditions.

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Helge Kristian Jenssen
Department of Mathematics
Penn State University, University Park
State College, PA 16802, USA
email: jenssen@math.psu.edu
Charis Tsikkou
Department of Mathematics
West Virginia University
Morgantown, WV 26506, USA
email: tsikkou@math.wvu.edu

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