# Homework Help: Commutator Problem

1. Apr 14, 2015

### Calu

1. The problem statement, all variables and given/known data

Let the commutator [A,B] = cI, I the identity matrix and c some arbitrary constant.

Show [A,Bn] = cnBn-1

2. Relevant equations

[A,B] = AB - BA

3. The attempt at a solution

So I have started off like this:

[A,Bn] = ABn - BnA = cI

I'm not sure where to go from here.

2. Apr 14, 2015

### HallsofIvy

I think you are misunderstanding the question. [A,B^n] = AB^n - B^nA = cI is NOT in general true. You seem to be thinking that "AB- BA= cI" is to be true for all A, B. It is not. In this problem AB- BA= cI is true for this specific A and B.

You are told that AB- BA= cI. So [A, B^2]= AB^2- B^2A= AB^2- BAB+ BAB- B^2A= (AB- BA)B+ B(AB- BA)= cB+ Bc= 2cB. etc. Use proof by induction.

3. Apr 14, 2015

### Dick

For any operators you have the identity $[D,EF]=[D,E]F+E[D,F]$ that's handy and it's easy to prove. The case $n=1$ is obvious, so now try $n=2$. Write $[A,B^2]$ as $[A,BB]$. For the general case think about induction.