# Homework Help: Special Cases of the BCH Identity

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1. Sep 16, 2018 at 11:23 AM

### WWCY

1. The problem statement, all variables and given/known data
Hi all, I'm having trouble working on the following problem. Any assistance will be greatly appreciated.

Here, the capital letters stand for Position and Momentum operators while the $x', p'$ stand for eigenvalues.

2. Relevant equations

3. The attempt at a solution

a) and b)

It seems that a) and b) can be written in the form of
$$e^{\frac{i}{\hbar}x'P}e^{\frac{i}{\hbar}p'X}$$
which I can then express in the form of the BCH identity
$$e^{\frac{i}{\hbar}x'P}e^{\frac{i}{\hbar}p'X} = e^{\frac{i}{\hbar}x'P + \frac{i}{\hbar}p'X + \frac{i}{2\hbar}x'p' + 0 + O'}$$
where I denote the other terms by $O'$. If $O' = 0$, I obtain b), if $O' = \frac{i}{2\hbar}x'p'$, I get a). Is this what the question was getting at? It does seem too convenient.

c)

Thanks in advance for any help!

2. Sep 21, 2018 at 7:00 PM

### PF_Help_Bot

Thanks for the thread! This is an automated courtesy bump. Sorry you aren't generating responses at the moment. Do you have any further information, come to any new conclusions or is it possible to reword the post? The more details the better.

3. Sep 21, 2018 at 9:42 PM

### nrqed

It is not the best terminology to say that you express something "in the form of the BCH identity". People don't usually put it this way. What you have to do is to prove in each case that the left hand side is equal to the rhs, and the BCH identity can help you demonstrate this. So you should start from the expression on one side and end up on the expression on the other side, using any identity or algebraic manipulation you can do.

Focus on (a). Start from the left side and show that you get to the right side. This is almost what you did, but you did not complete your proof. You wrote

$$e^{\frac{i}{\hbar}x'P}e^{\frac{i}{\hbar}p'X} = e^{\frac{i}{\hbar}x'P + \frac{i}{\hbar}p'X + \frac{i}{2\hbar}x'p' + 0 + O'}$$

Now you must determine $O'$. You cannot assume or choose the form it has. It has a unique expression and you must determine it. Once you will have shown what $O'$ is in this expression, you will have proven (a). Then we can discuss (b).