“…A different but related problem, concerns the existence of bound states in open geometries, that are not necessarily deformed, but contain local heterogeneities. For the solvable case of an infinite, straight, two-dimensional waveguide, containing a segment of different density, Wang [16] has proved the existence of bound states. Moreover, at least in two dimensions, the problem of studying the spectrum of a deformed waveguide can be reduced to the problem of studying the spectrum of a heterogeneous straight waveguide, by using a suitable conformal map which transforms one domain into the other.…”