Spectral Theory for Schrödinger Operators on Compact Metric Graphs with \(\delta \) and \(\delta '\) Couplings: A Survey

Spectral properties of Schrödinger operators on compact metric graphs are studied, and special emphasis is put on differences in the spectral behavior between different classes of vertex conditions. We survey recent results especially for \(\delta \) and \(\delta '\) couplings and demonstrate the spectral properties on many examples. Among other things, properties of the ground state eigenvalue and eigenfunction and the spectral behavior under various perturbations of the metric graph or the vertex conditions are considered.