Recently Neuberger, Zettl and
Loud have revived interest in self-adjoint boundary value problems with interior
point boundary conditions. All three have derived their results from rather extensive
study of the Green’s function associated with the nonhomogeneous problem. They
require G(x,ξ) = G(ξ,x)∗.
Rather than approach the problem via the Green’s function, this article considers
the problem as that of a differential operator in a Hilbert space, derives the adjoint
operator, whose domain specifies the adjoint boundary conditions, and then produces
necessary and sufficient conditions for self-adjointness.
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