PlanetMath (more info)
 Math for the people, by the people.
Encyclopedia | Requests | Forums | Docs | Wiki | Random | RSS  
Login
create new user
name:
pass:
forget your password?
Main Menu
Owner confidence rating: Very high Entry average rating: No information on entry rating
Baker-Campbell-Hausdorff formula(e) (Definition)

Given a linear operator $ A$, we define:

$\displaystyle \exp{A} := \sum_{k=0}^{\infty} \frac{1}{k!}A^k.$ (1)

It follows that
$\displaystyle \frac{\partial}{\partial \tau} e^{\tau A} = A e^{\tau A}= e^{\tau A} A.$ (2)

Consider another linear operator $ B$. Let $ B(\tau)=e^{\tau A} B e^{-\tau A}$. Then one can prove the following series representation for $ B(\tau)$:
$\displaystyle B(\tau)= \sum_{m=0}^{\infty} \frac{{\tau}^m}{m!}B_m,$ (3)

where $ B_m =[A,B]_m := [A,[A,B]_{m-1}]$ and $ B_0:=B$. A very important special case of eq. (3) is known as the Baker-Campbell-Hausdorff (BCH) formula. Namely, for $ \tau =1$ we get:
$\displaystyle e^A \; B e^{-A} = \sum_{m=0}^{\infty} \frac{1}{m!} B_m.$ (4)

Alternatively, this expression may be rewritten as
$\displaystyle [B,e^{-A}] = e^{-A} \left( [A,B]+\frac{1}{2} [A,[A,B]] + \cdots \right),$ (5)

or
$\displaystyle [e^A,B] = \left( [A,B]+\frac{1}{2} [A,[A,B]] + \cdots \right) e^A.$ (6)

There is a descendent of the BCH formula, which often is also referred to as BCH formula. It provides us with the multiplication law for two exponentials of linear operators: Suppose $ [A,[A,B]] = [B,[B,A]] = 0$. Then,
$\displaystyle e^A e^B = e^{A+B} e^{\frac{1}{2}[A,B]}.$ (7)

Thus, if we want to commute two exponentials, we get an extra factor
$\displaystyle e^A e^B = e^B e^A e^{[A,B]}.$ (8)



"Baker-Campbell-Hausdorff formula(e)" is owned by Mathprof. [ full author list (2) | owner history (1) ]
(view preamble)

View style:

Other names:  BCH formula, Baker-Campbell-Hausdorff formula, Baker-Campbell-Hausdorff formulae
Log in to rate this entry.
(view current ratings)

Cross-references: factor, exponentials, multiplication, expression, representation, series, linear operator
There are 2 references to this entry.

This is version 10 of Baker-Campbell-Hausdorff formula(e), born on 2003-06-04, modified 2008-02-11.
Object id is 4321, canonical name is BakerCampellHausdorffFormulae.
Accessed 26279 times total.

Classification:
AMS MSC47A05 (Operator theory :: General theory of linear operators :: General )

Pending Errata and Addenda
None.
[ View all 4 ]
Discussion
Style: Expand: Order:
forum policy

No messages.

Interact
post | correct | update request | add derivation | add example | add (any)