# Show that [A, F(B)] = [A, B]F'(B)

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1. Feb 29, 2016

### Jon.G

1. The problem statement, all variables and given/known data
A and B commute with their commutator, C=[A,B]
Show that [A, F(B)] = [A, B]F'(B)
F(B) = ∑n=0fnBn

2. Relevant equations
[A,B] = AB - BA
[A,BC] = [A,B]C + B[A,C]

3. The attempt at a solution
So all I can think to do is:
[A,Bn] = [A,BBn-1] = [A,B]Bn-1 + B[A,Bn-1]
=ABBn-1-BABn-1+BABn-1-BBn-1A
=ABBn-1 - BBn-1A
and this seems to be a dead end? Just loops back around to ABn - BnA
If this is along the right track, whereabouts would I go next? If this is not the way to start it, how would I go about beginning this problem?
And even if this is along the right lines, I've ignored the sum here as I'm not quite sure how that affects the whole thing.

2. Feb 29, 2016

### Dick

Start by showing the case where f(B)=B^2. You should get [A,f(B)]=[A,B]2B. Which is [A,B]f'(B). Then try f(B)=B^3. For the general case use induction to prove if f(B)=B^n, then [A,B^n]=[A,B]nB^(n-1). Which is [A,B]f'(B). Now deal with the sum.

3. Mar 1, 2016

### Jon.G

Ooh ok thanks.
So I went through it with n=2,3 and 4 and [A, F(B)] = [A, B]F'(B) was shown for each of them.
For the sum is it simply that when n is any value, fn is a constant so that
(for example)
[A,f2B2] = [A, f2B]B + f2B[A, B]
f2 a constant and can be taken outside of [] as it effect both sides the same so (and as B commutes with [A, B])
[A, f2B]B + f2B[A, B] = [A,B]f2B + [A,B]f2B
=[A,B]2f2B = [A, B]F'(B)
?
Is this how it's shown?

4. Mar 1, 2016

### Dick

Yes, the Lie bracket is linear over scalars, so $[A,cf(B)+dg(B)]=c[A,f(B)]+d[A,g(B)]$. That's how you take care of the sum. It's good that you've shown it works for n=2,3 and 4. To show it's true for general $n$ that $[A,B^n]=[A,B]nB^{n-1}$ there's a general procedure called mathematical induction. You assume it's true for $n$ and then show that it must be true for $n+1$. Since you've already shown that it works for small values of n, that shows it works for all values of n. A less formal argument is to show, for example, that $[A,B^3]=[A,BBB]=[A,B]BB+B[A,B]B+BB[A,B]$ and then say this pattern holds for all n.

Last edited: Mar 1, 2016