On self-adjoint boundary conditions for singular Sturm-Liouville operators bounded from below

Abstract

We extend the classical boundary values \begin{align*} & g(a) = - W(u_{a}(λ_0,.), g)(a) = \lim_{x \downarrow a} \frac{g(x)}{\hat u_{a}(λ_0,x)}, \\ &g^{[1]}(a) = (p g')(a) = W(\hat u_{a}(λ_0,.), g)(a) = \lim_{x \downarrow a} \frac{g(x) - g(a) \hat u_{a}(λ_0,x)}{u_{a}(λ_0,x)} \end{align*} for regular Sturm-Liouville operators associated with differential expressions of the type $\tau =r(x{)}^{-1}[-(d/dx)p(x)(d/dx)+q(x)]$ for a.e. $x\in [a,b]\subset \mathbb{R}$, to the case where $\tau$ is singular on $(a,b)\subseteq \mathbb{R}$ and the associated minimal operator $T_{min}$ is bounded from below. Here $u_a({\lambda }_0,\cdot )$ and ${\hat{u}}_a({\lambda }_0,\cdot )$ denote suitably normalized principal and nonprincipal solutions of $\tau u={\lambda }_{0}u$ for appropriate ${\lambda }_0\in \mathbb{R}$, respectively. We briefly discuss the singular Weyl-Titchmarsh-Kodaira $m$-function and finally illustrate the theory in some detail with the examples of the Bessel, Legendre, and Kummer (resp., Laguerre) operators.

38 pages