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sayebms
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Homework Statement
Suppose a linear operator L satisfies <A|L|A> = 0 for every state A. Show that then all matrix elements <B|L|A> = 0, and hence L = 0.
Homework Equations
##<A|L|A>=L_{AA} and <B|L|A>=L_{BA}##
The Attempt at a Solution
It seems very straight forward and I don't know how to prove it but here is what I have tried:
##<B|L|A> \to##Using resolution of Identity ##\to \sum_{A} <B|A><A|L|A> \to <B|L|A>=0##
Is it right or do I need to write more.
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