  Given a densely defined skew-symmetric operators A 0 on a real or complex
Hilbert space V , we parametrize all m-dissipative extensions in terms of
contractions $\Phi$ : H-$\rightarrow$ H + , where Hand H + are Hilbert spaces
associated with a boundary quadruple. Such an extension generates a unitary C
0-group if and only if $\Phi$ is a unitary operator. As corollary we obtain the
parametrization of all selfadjoint extensions of a symmetric operator by
unitary operators from Hto H +. Our results extend the theory of boundary
triples initiated by von Neumann and developed by V. I. and M. L. Gorbachuk, J.
Behrndt and M. Langer, S. A. Wegner and many others, in the sense that a
boundary quadruple always exists (even if the defect indices are different in
the symmetric case).
