Let {V(t)vertical bar t is an element of [0, infinity)} be a one-parameterstronglycontinuous semigroup of contractions on a separable Hilbert space and let V(-t) : = V*(t) for l is an element of [0, infinity). It is...
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Let {V(t)vertical bar t is an element of [0, infinity)} be a one-parameterstronglycontinuous semigroup of contractions on a separable Hilbert space and let V(-t) : = V*(t) for l is an element of [0, infinity). It is shown that if V(t) is a partial isometry for all t is an element of [-t(0) , t(0)], t(0) > 0, then the pointwise two-sided derivative of V(t) exists on a dense domain of vectors. This derivative B is necessarily a densely defined symmetric operator. This result can be viewed as a generalization of Stone's theorem for one-parameterstronglycontinuous unitary groups, and is used to establish sufficient conditions for a self-adjoint operator on a Hilbert space K to have a symmetric restriction to a dense linear manifold of a closed subspace H subset of K. A large class of examples of such semigroups consisting of the compression of the unitary group generated by the operator of multiplication by the independent variable in K := circle plus(n)(i=1) L-2 (R) to certain model subspaces of the Hardy space of n-compenent vector valued functions which are analytic in the upper half plane is presented.
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