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This question already has an answer here:

Found this statement in Quantum Mechanics by David McIntyre and was wondering how this is proven.

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marked as duplicate by user191954, Cosmas Zachos, Aaron Stevens, Jon Custer, ZeroTheHero Oct 14 '18 at 5:01

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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    $\begingroup$ Can you give a direct quote and some more context? $\endgroup$ – Aaron Stevens Oct 13 '18 at 4:35