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If we want to work out the expectation of:

<0|T(φ_{1}φ_{2})|0>

ie. <0|<0|T(φ_{1}φ_{2})|0>|0>

apparently it is acceptable to pull out the <0|T(φ_{1}φ_{2})|0>:

So <0|<0|T(φ_{1}φ_{2})|0>|0>=<0|T(φ_{1}φ_{2})|0><0|I|0>

I do realise this is a really stupid question, but I want to be 100% sure. Is this simply because an expectation is always a constant, not an operator which acts on a state? Can you always pull out an expectation in this way?

Thanks.

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# The expectation of an expection (relating to Wick's Theorem)

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