“…Schrödinger operators with singular δ-type interactions supported on discrete sets, curves and surfaces are used for the description of quantum mechanical systems with a certain degree of idealization. The spectral properties of Schrödinger operators with δ and δ ′ -interactions were investigated in numerous mathematical and physical articles in the recent past; we mention only [BN11, KM10,MS12,O10] for interactions on point sets, [CK11, EI01, EK08, EN03, EP12, K12, KV07] on curves, and [AKMN13, BLL13, EF09, EK03] for interactions on surfaces. For a survey and further references we refer the reader to [E08] and to the standard monograph [AGHH].…”