Selfadjoint extensions of relations whose domain and range are orthogonal

Abstract

The selfadjoint extensions of a closed linear relation $R$ from a Hilbert space ${\mathfrak{H}}_1$ to a Hilbert space ${\mathfrak{H}}_2$ are considered in the Hilbert space ${\mathfrak{H}}_1\oplus {\mathfrak{H}}_2$ that contains the graph of $R$. They will be described by $2\times 2$ blocks of linear relations and by means of boundary triplets associated with a closed symmetric relation $S$ in ${\mathfrak{H}}_1\oplus {\mathfrak{H}}_2$ that is induced by $R$. Such a relation is characterized by the orthogonality property $\mathrm{dom}S\perp \mathrm{ran}S$ and it is nonnegative. All nonnegative selfadjoint extensions $A$, in particular the Friedrichs and Kre\uın-von Neumann extensions, are parametrized via an explicit block formula. In particular, it is shown that $A$ belongs to the class of extremal extensions of $S$ if and only if $\mathrm{dom}A\perp \mathrm{ran}A$. In addition, using asymptotic properties of an associated Weyl function, it is shown that there is a natural correspondence between semibounded selfadjoint extensions of $S$ and semibounded parameters describing them if and only if the operator part of $R$ is bounded.

26 pages