a linear relation $T$ in $L$. For a linear relation $T:L_0\to L_1$ we denote by $\dom T,\,\ran T,
\,\ker T$ and $\mul T$ the domain, range, kernel and the multivalued part of $T$ respectively. The inverse $T^{-1}$ is a linear relation from $L_1$ to $L_0$ defined by $T^{-1}=\{\{f', f\}:\{f,f'\}\in T\}$.

Assume now that $\cH_0$ and $\cH_1$ are Hilbert spaces. Then the linear space $\cH_0\times\cH_1$ with the inner product $(\{f,f'\}, \{g,g'\})_{\cH_0
\oplus\cH_1} =(f,g)_{\cH_0}+(f',g')_{\cH_1}$ is a Hilbert space $\cH_0\oplus\cH_1$. The set of all closed linear relations from $\cH_0$ to $\cH_1$ (in $\cH$) will be denoted by $\C (\cH_0,\cH_1)$ ($\C(\cH)$). A closed linear operator $T$ from $\cH_0$ to $\cH_1$ is identified with its graph $\text {gr}\, T\in\CA$. For a linear relation $T:\cH_0\to \cH_1$ we denote by $T^*(\in\CB)$ the adjoint relation

In the case $T\in\CA$ we write $0\in \rho (T)$ if $\ker T=\{0\}$ and $\ran
T=\cH_1$, or equivalently if $T^{-1}\in [\cH_1,\cH_0]$; $0\in \hat\rho (T)$ if   $\ker T=\{0\}$ and  $\ran (T)$ is a closed subspace in $\cH_1$. For a linear relation $T\in \C(\cH)$ we denote by $\rho (T):=\{\l \in \bC:\ 0\in \rho
(T-\l)\}$ and $\hat\rho (T)=\{\l \in \bC:\ 0\in \hat\rho (T-\l)\}$ the resolvent set and the set of regular type points of $T$ respectively.

For 