- #1

Berny

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## Homework Statement

|phi (n)> being eigen states of hermitian operator H ( H could be for example the hamiltonian

of anyone physical system ). The states |phi (n)> form an orthonormal discrete basis.

The operator U(m,n) is defined by:

U(m,n)= |phi(m)><phi(n)|

Calculate the commutator:

[H,U(m,n)]

( this is part of the first problem in Cohen, Tannoudji, Diu, Laloe textbook in quantum mechanics.)

## The Attempt at a Solution

[itex]\[/itex][HU-UH] (ψ) = H|phi(m)><phi (n)|ψ> - |phi (m)><phi(n)| H| ψ>

= <phi(n)|ψ> H |phi (m>) - |phi (m><phi(n)| <ψ | H

and then ? i did not find symbol phi.