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I am reading a book in which the unitary evolution operator is

[tex]U = \exp(-i H/\hbar)[/tex]

where H is the given Hamiltonian. But in another book, I found that the evolution operator is general given as

[tex]U = \exp(-i \int H(t) dt / \hbar)[/tex]

which one is correct and why there are two expression? Thanks

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# About unitary operator

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