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We consider Schrdinger operators in L2(R3) with a singular interaction supported by a finite curve . We present a proper definition of the operators and study their properties, in particular, we show that the discrete spectrum can be empty if is short enough. If it is not the case, we investigate properties of the eigenvalues in the situation when the curve has a hiatus of length 2. We derive an asymptotic expansion with the leading term which a multiple of ln.

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Quantum networks are often modelled using Schrdinger operators on metric graphs. To give meaning to such models one has to know how to interpret the boundary conditions which match the wave functions at the graph vertices. In this article we give a survey, technically not too heavy, of several recent results which serve this purpose. Specifically, we consider approximations by means of fat graphsin other words, suitable families of shrinking manifoldsand discuss convergence of the spectra and resonances in such a setting.

Let A and A be non-negative self-adjoint operators in a separable Hilbert space such that its form sum A is densely defined. It is shown that the Trotter product formula holds for imaginary times in the A -norm, that is, one has%%\begin {displaymath}