Published online by Cambridge University Press: 14 March 2014
We consider operators $ - \Delta + X $, where
$ X $ is a constant vector field, in a bounded domain, and show spectral
instability when the domain is expanded by scaling. More generally, we consider
semiclassical elliptic boundary value problems which exhibit spectral instability for
small values of the semiclassical parameter
$h$, which should be thought of as the reciprocal of the Péclet
constant. This instability is due to the presence of the boundary: just as in the
case of
$ - \Delta + X $, some of our operators are normal when considered on
$\mathbb{R}^d$. We characterize the semiclassical pseudospectrum of such problems
as well as the areas of concentration of quasimodes. As an application, we prove a
result about exit times for diffusion processes in bounded domains. We also
demonstrate instability for a class of spectrally stable nonlinear evolution problems
that are associated with these elliptic operators.