Geodesic
The General Differential Operators Generated by a Quasi-Differential Expressions with their Interior Singular Points
Serdica Mathematical Journal, Tome 25 (1999) no. 3, pp. 207-240.

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The general ordinary quasi-differential expression M of n-th order with complex coefficients and its formal adjoint M + are considered over a regoin (a, b) on the real line, −∞ ≤ a b ≤ ∞, on which the operator may have a finite number of singular points. By considering M over various subintervals on which singularities occur only at the ends, restrictions of the maximal operator generated by M in L2|w (a, b) which are regularly solvable with respect to the minimal operators T0 (M ) and T0 (M + ). In addition to direct sums of regularly solvable operators defined on the separate subintervals, there are other regularly solvable restrications of the maximal operator which involve linking the various intervals together in interface like style.
Keywords: Quasi-Differential Expressions, Regular and Singular End-Points, Regularly Solvable Operators, Hilbert Space, Boundary Conditions 
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El-sayed Ibrahim, Sobhy. The General Differential Operators Generated by a Quasi-Differential Expressions with their Interior Singular Points. Serdica Mathematical Journal, Tome 25 (1999) no. 3, pp. 207-240. http://geodesic.mathdoc.fr/item/SMJ2_1999_25_3_a2/