Abstract
The second-order symmetric Sturm-Liouville differential
expressions τ1,τ2,…,τn with real
coefficients are considered on the interval I=(a,b), −∞≤a<b≤∞. It is shown that the characterization
of singular selfadjoint 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 by
Everitt and Zettl (1977), Hinton, Krall, and Shaw (1987),
Ibrahim (1999), Krall and Zettl (1988), Lee (1975/1976), and
Naimark (1968).