Below is the ascii version of the abstract for 96-46.
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Davies E.B., Parnovski L.
Trapped modes in acoustic waveguides.
ABSTRACT. We consider the Laplace operator acting on the infinite cylinder
with an obstacle inside it. Boundary conditions are assumed to
be Neumann. We study the existence of (embedded) eigenvalues in
two cases. For two-dimentional symmetric obstacle we give both
necessary and sufficient conditions for the existence of an
eigenvalue. For thin nonsymmetrical obstacle parallel to the
axis of the cylinder we prove that an eigenvalue always exists.