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Stone's theorem on one-parameter unitary groups
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Stone's theorem on one-parameter unitary groups : ウィキペディア英語版
Stone's theorem on one-parameter unitary groups
In mathematics, Stone's theorem on one-parameter unitary groups is a basic theorem of functional analysis that establishes a one-to-one correspondence between self-adjoint operators on a Hilbert space and one-parameter families
: (U_)_ \in \mathbf, ~ \xi \in H: \qquad \lim_ \xi = U_: \qquad U_ = U_ U_.
Such one-parameter families are ordinarily referred to as strongly continuous one-parameter unitary groups.
The theorem was proved by , and showed that the requirement that (U_)_{t \in \mathbf{R}} be strongly continuous can be relaxed to say that it is merely weakly measurable, at least when the Hilbert space is separable.
This is a very stunning theorem, as it allows to define the derivative of the mapping , which is only supposed to be continuous. It is also related to the theory of Lie groups and Lie algebras.
== Formal statement ==
Let (U_)_: \qquad U_ = e^.
Conversely, let be a (not necessarily bounded) self-adjoint operator on a Hilbert space . Then the one-parameter family (U_)_: \qquad U_ := e^
is a strongly continuous one-parameter group.
The infinitesimal generator of (U_)_)_): \qquad \rho(f) := \int_ \, dt,
:and then extending to all of by continuity.
* Use the Fourier transform to obtain a non-degenerate
*-representation of on .
* By the
Riesz-Markov Theorem, gives rise to a projection-valued measure on that is the resolution of the identity of a unique self-adjoint operator , which may be unbounded.
* Then is the infinitesimal generator of (U_)_),\star) . Then is defined to be the enveloping C
*-algebra of (L^1(\mathbf),\star) , i.e., its completion with respect to the largest possible C
*-norm. It is a non-trivial fact that, via the Fourier transform, is isomorphic to . A result in this direction is the Riemann-Lebesgue Lemma, which says that the Fourier transform maps to .

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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