# Operator algebra with commutators

## Homework Statement

Consider the operator A and its Hermitian adjoint A*.

If [A,A*] = 1, evaluate: [A*A,A]

## Homework Equations

standard rules of linear algebra, operator algebra and quantum mechanics

## The Attempt at a Solution

[A,A*] = AA* - A*A = 1

A*A = (1+AA*)

[A*A,A] = AA*A - A*AA = ???????

How do I even start?

## Answers and Replies

Hi chill_factor,

If we expand [A*A,A], we obtain A*AA-AA*A. Do you see a way to factor this that might allow you to use the [A,A*] commutator?

Hi chill_factor,

If we expand [A*A,A], we obtain A*AA-AA*A. Do you see a way to factor this that might allow you to use the [A,A*] commutator?

Thank you for answering. I think I see it. There is an A on the right side of both the A*AA and AA*A operators so A*AA - AA*A = (A*A-AA*)A = -[A,A*]A = -A. Is that right?

I guess my mistake was writing the commutator out incorrectly.

Thank you for answering. I think I see it. There is an A on the right side of both the A*AA and AA*A operators so A*AA - AA*A = (A*A-AA*)A = -[A,A*]A = -A. Is that right?

I guess my mistake was writing the commutator out incorrectly.

Yep. You've got it. Nice work.