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ELECTRONIC JOURNAL OF DIFFERENTIAL EQUATIONS, 2013 (SCI-Expanded)
This article describes all selfadjoint extensions of the minimal operator generated by a linear singular multipoint symmetric differential-operator expression for first order in the direct sum of Hilbert spaces of vector-functions. This description is done in terms of the boundary values, and it uses the Everitt-Zettl and the Calkin-Gorbachuk methods. Also the structure of the spectrum of these extensions is studied.