We prove an abstract criterion stating resolvent convergence in the case of
operators acting in different Hilbert spaces. This result is then applied to
the case of Laplacians on a family X_\eps of branched quantum waveguides.
Combining it with an exterior complex scaling we show, in particular, that the
resonances on X_\eps approximate those of the Laplacian with ``free''
boundary conditions on X0, the skeleton graph of X_\eps.Comment: 48 pages, 1 figur