Brown, B. Malcolm; Langer, Heinz; Tretter, Christiane (2019). Compressed resolvents and reduction of spectral problems on star graphs. Complex analysis and operator theory, 13(1), pp. 291320. Birkhäuser 10.1007/s1178501807936

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In this paper a twostep reduction method for spectral problems on a star graph with n+1 edges e₀, e₁, .... , eₙ and a selfadjoint matching condition at the central vertex v is established. The first step is a reduction to the problem on the single edge e₀ but with an energy depending boundary condition at v. In the second step, by means of an abstract inverse result for Qfunctions, a reduction to a problem on a path graph with two edges e₀, ẽ₁ joined by continuity and Kirchhoff conditions is given. All results are proved for symmetric linear relations in an orthogonal sum of Hilbert spaces. This ensures wide applicability to various different realizations, in particular, to canonical systems and Krein strings which include, as special cases, Dirac systems and Stieltjes strings. Employing two other key inverse results by de Branges and Krein, we answer e.g. the following question: If all differential operators are of one type, when can the reduced system be chosen to consist of two differential operators of the same type?
Item Type: 
Journal Article (Original Article) 

Division/Institute: 
08 Faculty of Science > Department of Mathematics and Statistics > Institute of Mathematics 
UniBE Contributor: 
Langer, Heinz and Tretter, Christiane 
Subjects: 
500 Science > 510 Mathematics 
ISSN: 
16618254 
Publisher: 
Birkhäuser 
Language: 
English 
Submitter: 
Michel Arthur Bik 
Date Deposited: 
06 Aug 2019 14:44 
Last Modified: 
25 Oct 2019 03:37 
Publisher DOI: 
10.1007/s1178501807936 
BORIS DOI: 
10.7892/boris.132259 
URI: 
https://boris.unibe.ch/id/eprint/132259 