  In this paper we address the problem of wave dynamics in presence of
concentrated nonlinearities. Given a vector field $V$ on an open subset of
$\CO^n$ and a discrete set $Y\subset\RE^3$ with $n$ elements, we define a
nonlinear operator $\Delta_{V,Y}$ on $L^2(\RE^3)$ which coincides with the free
Laplacian when restricted to regular functions vanishing at $Y$, and which
reduces to the usual Laplacian with point interactions placed at $Y$ when $V$
is linear and is represented by an Hermitean matrix. We then consider the
nonlinear wave equation $\ddot \phi=\Delta_{V,Y}\phi$ and study the
corresponding Cauchy problem, giving an existence and uniqueness result in the
case $V$ is Lipschitz. The solution of such a problem is explicitly expressed
in terms of the solutions of two Cauchy problem: one relative to a free wave
equation and the other relative to an inhomogeneous ordinary differential
equation with delay and principal part $\dot\zeta+V(\zeta)$. Main properties of
the solution are given and, when $Y$ is a singleton, the mechanism and details
of blow-up are studied.
