What is Cn in quantum mechanics?
What is the context in which you saw this?
I saw in derivation of "sudden approximation"
Be more specific. Cn is just a symbol that is highly context dependent. There is no standard meaning of Cn.
Remember my tip to post links to your reading -- that will help us a lot in answering your question.
Does this old PF thread help?
ι∅(t) = ∑CneiEnt/h
The equation contains a sum of terms indexed by the integer n, and n ranges from 1 to infinity.
Cn is the complex coefficient in term n.
You can see similar notation in http://lpsa.swarthmore.edu/Fourier/Series/ExFS.html
Thanks for telling
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