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Diophantine approximation, Bessel functions and radially symmetric periodic solutions of semilinear wave equations in a ball
Authors:J Berkovits  J Mawhin
Institution:Department of Mathematics, University of Oulu, Oulu, Finland ; Université Catholique de Louvain, Institut Mathématique, B-1348 Louvain-la-Neuve, Belgium
Abstract:

The aim of this paper is to consider the radially-symmetric periodic-Dirichlet problem on $0,T] \times B^na]$ for the equation

\begin{displaymath}u_{tt} - \Delta u = f(t,\vert x\vert,u),\end{displaymath}

where $\Delta$ is the classical Laplacian operator, and $B^na]$ denotes the open ball of center $0$ and radius $a$ in ${\mathbb R}^n.$ When $\alpha = a/T$ is a sufficiently large irrational with bounded partial quotients, we combine some number theory techniques with the asymptotic properties of the Bessel functions to show that $0$ is not an accumulation point of the spectrum of the linear part. This result is used to obtain existence conditions for the nonlinear problem.

Keywords:Diophantine approximations  Bessel functions  wave equation  periodic solutions
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