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## Homework Statement

We have an hermitian operator A and another operator B, and let's say they don't commute, i.e. [A,B] = cI (I is identity). So, if we take a normalized wavefunction |a> that is eigenfunction of the operator A so that A|a> = a|a>, we should have

<a|[A,B]|a> = c.

But if i write

<a|AB|a> - <a|BA|a>

and, since A is hermitian, i make it act on the bra for the first term and on the ket for the second one i get

a<a|B|a> - a<a|B|a> = 0.

I really don't see where is the problem...