  We consider the inverse boundary value problem of determining the potential
$q$ in the equation $\Delta u + qu = 0$ in $\Omega\subset\mathbb{R}^n$, from
local Cauchy data. A result of global Lipschitz stability is obtained in
dimension $n\geq 3$ for potentials that are piecewise linear on a given
partition of $\Omega$. No sign, nor spectrum condition on $q$ is assumed, hence
our treatment encompasses the reduced wave equation $\Delta u + k^2c^{-2}u=0$
at fixed frequency $k$.
