Schubert, CarstenCarstenSchubertSeifert, ChristianChristianSeifertVoigt, JürgenJürgenVoigtWaurick, MarcusMarcusWaurick2021-11-082021-11-082015-04-15Mathematische Nachrichten 288 (14/15): 1776-1785 (2015-10-01)http://hdl.handle.net/11420/10805We generalize the notion of Lagrangian subspaces to self-orthogonal subspaces with respect to a (skew-) symmetric form, thus characterizing (skew-)self-adjoint and unitary operators by means of self-orthogonal subspaces. By orthogonality preserving mappings, these characterizations can be transferred to abstract boundary value spaces of (skew-)symmetric operators. Introducing the notion of boundary systems we then present a unified treatment of different versions of boundary triples and related concepts treated in the literature. The application of the abstract results yields a description of all (skew-)self-adjoint realizations of Laplace and first derivative operators on graphs.en1522-2616201514/1517761785Wiley-VCH(skew-)self-adjoint operators05C9935Q9947B25Boundary tripleQuantum graphsMathematikJournal Article10.1002/mana.201500054Other