Mark M. Malamud, Hagen Neidhardt
Sturm-Liouville boundary value problems with operator potentials and 
unitary equivalence
(563K, pdf)

ABSTRACT.  Consider the minimal Sturm-Liouville operator $A = 
A_{
m min}$ generated by the differential expression 
$\mathcal{A} := -rac{d^2}{dt^2} + T$ 
in the Hilbert space $L^2(\mathbb{R}_+,\mathcal{H})$ where $T = T^*\ge 0$ in $\mathcal{H}$. We investigate the absolutely continuous parts of different self-adjoint realizations of $\mathcal{A}$. In particular, we show that Dirichlet and Neumann realizations, $A^D$ and $A^N$, are absolutely continuous and unitary equivalent to each other and to the 
absolutely continuous part of the Krein realization. Moreover, 
if $\inf\sigma_{
m ess}(T) = \inf\sigma(T) \ge 0$, then the 
part $\widehat{A}^{ac}E_{\widehat{A}}(\sigma(A^D))$ of any self-adjoint realization $\widehat{A}$ of $\mathcal{A}$ is unitarily equivalent to $A^D$. In addition, we prove that the absolutely continuous part 
$\widehat{A}^{ac}$ of any realization $\widehat{A}$ is unitarily 
equivalent to $A^D$ provided that the resolvent difference 
$(\widehat{A} - i)^{-1}- (A^D - i)^{-1}$ is compact. The abstract 
results are applied to elliptic differential expression in the half-space.