\documentclass{article} \usepackage{amsmath} \usepackage{amsfonts} \usepackage{amsthm} \newtheorem{theorem}{Theorem} \begin{document} \begin{theorem} Let $S_t$ be a strongly continuous semigroup of partial isometries in a Hilbert space $H$. Then $H$ can be decomposed into a direct sum of invariant with respect to $S_t$, $S_t^*$, $t\ge0$, subspaces, $H=H_0\oplus H_+\oplus H_- \oplus H_1$ such that: in $H_1$ the semigroup $S_t$, $t\ge0$ is unitary, i.e., all operators $S_t$ are unitary\textup; \end{theorem} \begin{proof} Indeed, \end{proof} \end{document} %%% Local Variables: %%% mode: latex %%% TeX-master: t %%% End:

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