# [note] useful commutator relations

### story

This is going to be a quick post. Trying to look up common commutation relationships is a pain. I’m going to compile a list of these relationships that I’ve found useful. This post should be treated as a reference. I will likely update this post as time goes on.

### quick introduction

Unfortunately, a lot of useful algebras are not commutative. The commutator describes how close two elements are to commuting and often encapsulates some interesting properties.

\[ [A,B] = AB - BA \]

For the rest of this post, capital letters \(A\), \(B\), \(C\) etc. will be used to denote elements of a non-commutative algebra and lowercase letters will be used to denote elements of the field.

### basic commutator algebra

Addition within the commutator, \[ [A+B,C]=[A,C]+[B,C] \]

Multiplication within the commutator, \[ [A,BC]=B[A,C]+[A,B]C \]

This one shows up sometimes when composing unitary transformations, \[ \frac{\partial}{\partial t}(e^{tA}Be^{-tA})=e^{tA}[A,B]e^{-tA} \]

### Baker-Campbell-Hausdorf (-lite)

The full BCH relationship is very general and unwieldy. Luckly, in a lot of situations the commutator commutes with the original elements truncating the BCH formula after the first commutator term. Small note: this might actually be known as the Zassenhaus formula, with the real BCH being its dual, but I think most people would identify this as the BCH relationship.

If \[ [A,[A,B]]=[B,[A,B]]=0 \] then, \[ e^{t(A+B)} = e^{tA}e^{tB}e^{-\frac{t^2}{2}[A,B]} \]

The two relationships below are also closely related to the BCH expansion,

\[ e^BAe^{-B}=A+[B,A] \]

\[ Ae^B = e^BA+[A,B]e^B \]

the first one is also known as the Hadamard lemma.

### Jacobi identity

Another one that shows up frequently is the Jacobi identity.

\[ [A,[B,C]] + [B,[C,A]] + [C,[A,B]] = 0 \]

Rewritten, the Jacobi identity can be seen as a generalization of associativity,

\[ [[A,B],C] = [A,[B,C]] - [B,[A,C]] \]

The standard cross product also satisfies the Jacobi identity (the lowercase letters below are elements of the vector space \(\mathbb{R}^3\)),

\[ (a \times b) \times c = a \times (b \times c) - b \times (a \times c) \]

### revision history

May 9 – started post