Symmetric operators with real defect subspaces of the maximal dimension. Applications to differential operators

Abstract: Let $\gH$ be a Hilbert space and let $A$ be a simple symmetric operator in $\gH$ with equal deficiency indices $d:=n_\pm(A)<\infty$. We show that if, for all $\l$ in an open interval $I\subset\bR$, the dimension of defect subspaces $\gN_\l(A)(=\Ker (A^*-\l))$ coincides with $d$, then every self-adjoint extension $\wt A\supset A$ has no continuous spectrum in $I$ and the point spectrum of $\wt A$ is nowhere dense in $I$. Application of this statement to differential operators makes it possible to generalize the known results by Weidmann to the case of an ordinary differential expression with both singular endpoints and arbitrary equal deficiency indices of the minimal operator. Moreover, we show in the paper, that an old conjecture by Hartman and Wintner on the spectrum of a self-adjoint Sturm - Liouville operator is not valid.