Abstract
In this paper; the second-order symmetric Sturm-liouville differential expressions tau(1), tau(2),..., tau(n) with real coefficients are considered on the interval I = (a, b), - infinity less than or equal to a less than or equal to b greater than or equal to infinity. It is shown that the characterization of singular self-adjoint boundary conditions involves the sesquilinear form associated with the product of Sturm-Liouville differential expressions and elements of the maximal domain of the product operators, and it is an exact parallel of the regular case. This characterization is an extension of those obtained in [6], [8], [11], [12], [14] and [15].