}), as: $\documentclass[12pt]{minimal}
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                \begin{document}$${\mathcal {O}}_{t} := e^{t {\mathcal {H}}^{\text {R}}} {\mathcal {O}} e^{-t{\mathcal {H}}^{\text {R}}}$$\end{document}$. Notice that $\documentclass[12pt]{minimal}
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                \begin{document}$${\mathcal {O}}_{t} \equiv {\mathcal {O}}(-it)$$\end{document}$, with $\documentclass[12pt]{minimal}
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                \begin{document}$${\mathcal {O}}(t)$$\end{document}$ the real-time evolution generated by $\documentclass[12pt]{minimal}
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                \begin{document}$${\mathcal {H}}^{\text {R}}$$\end{document}$. Given *n* operators $\documentclass[12pt]{minimal}
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