TURKISH JOURNAL OF MATHEMATICS, cilt.42, sa.4, ss.2071-2079, 2018 (SCI-Expanded)
In this paper, using the Calkin-Gorbachuk method, the general form of all self-adjoint operators generated by first order linear singular multipoint quasi-differential expressions in the direct sum of weighted Hilbert spaces of vector functions has been found. Later on, the geometry of the spectrum set of these type extensions was researched.