# Eikonal Blog

## 2010.01.28

### BCS (Baker–Campbell–Hausdorff) formula

Filed under: Uncategorized — Tags: , — sandokan65 @ 17:12

In this posting the matrix $C(t)$ is defined by $e^{C(t)}:\equiv e^{t(A+B)}$ where $A$ and $B$ are constant matrices.

The Baker–Campbell–Hausdorff theorem claims that:

$C(t) = B + \int_0^1 dt g(e^{t a} e^b ) A$,

where $g(z):\equiv \frac{\ln(z)}{z-1} = \sum_{m=0}^\infty \frac{(1-z)^m}{m+1}$, and the lower-case letters $a$ and $b$ represent the adjoint actions of the corresponding matrices $A$ and $B$ (e.g. $a X:\equiv ad(A) X :\equiv [A,X]$).

One is frequently seeing the following series expression:

$C(t) = t (A+B) + \frac{t^2}2 [A,B] + \frac{t^3}{12} ([[A.B],B]-[[A,B],A]) + \cdots = t (A+B) + \frac{t^2}2 a B + \frac{t^3}{12} (b^2 A + a^2 B) + \cdots$

Source: T1269 = A.N.Richmond “Expansion for the exponential of a sum of matrices”; Int. Journal of Control (issue and year unknown).