Phillips symmetric operators and their extensions

Let $S$ be a symmetric operator with equal defect numbers and let $\mathfrak{U}$ be a set of unitary operators in a Hilbert space $\mathfrak{H}$. The operator $S$ is called $\mathfrak{U}$-invariant if $US=SU$ for all $U\in\mathfrak{U}$. Phillips \cite{PH} constructed an example of $\mathfrak{U}$-invariant symmetric operator $S$ which has no $\mathfrak{U}$-invariant self-adjoint extensions. It was discovered that such symmetric operator has a constant characteristic function \cite{KO}. For this reason, each symmetric operator $S$ with constant characteristic function is called a \emph{Phillips symmetric operator}.