Home > Campbell-Hausdorff formula

z = ln(e^xe^y)

for non-commuting x and y.

Specifically, let G be a simply-connected Lie group with Lie algebra . Let

exp:

be the exponential map, defining

The general formula is given by:

Here

ad(A)B = [A,B].

In terms in the sum where , the last three factors should be interpreted as .

The first few terms are well-known:

There is no expression in closed form.

For a matrix Lie algebra the Lie algebra is the tangent space of the identity I, and the commutator is simply [X,Y] = XY - YX; the exponential map is the standard exponential map of matrices,

When we solve for Z in

e^Z = e^X e^Y,

we obtain a simpler formula:

.

We note that the first, second, third and fourth order terms are:

1 References

• L. Corwin & F.P Greenleaf (1990) Representation of nilpotent Lie groups and their applications, Part 1: Basic theory and examples,
  • (BooksEnthsiast.com)

2 External link

http://mathworld.wolfram.com/Baker-Campbell-HausdorffSeries.html