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## Associativity of the Baker-Campbell-Hausdorff formula

29 October, 2011 in expository, math.GR, math.RA | Tags: Baker-Campbell-Hausdorff formula, Lie algebras, Lie groups, local Lie groups | by Terence Tao | 5 comments

Let ${{\mathfrak g}}$ be a finite-dimensional Lie algebra (over the reals). Given two sufficiently small elements ${x, y}$ of ${{\mathfrak g}}$, define the right Baker-Campbell-Hausdorff-Dynkin law

$\displaystyle R_y(x) := x + \int_0^1 F_R( \hbox{Ad}_x \hbox{Ad}_{ty} ) y \ dt \ \ \ \ \ (1)$

where ${\hbox{Ad}_x := \exp(\hbox{ad}_x)}$, ${\hbox{ad}_x: {\mathfrak g} \rightarrow {\mathfrak g}}$ is the adjoint map ${\hbox{ad}_x(y) := [x,y]}$, and ${F_R}$ is the function ${F_R(z) := \frac{z \log z}{z-1}}$, which is analytic for ${z}$ near ${1}$. Similarly, define the left Baker-Campbell-Hausdorff-Dynkin law

$\displaystyle L_x(y) := y + \int_0^1 F_L( \hbox{Ad}_{tx} \hbox{Ad}_y ) x\ dt \ \ \ \ \ (2)$

where ${F_L(z) := \frac{\log z}{z-1}}$. One easily verifies that these expressions are well-defined (and depend smoothly on ${x}$ and ${y}$) when ${x}$ and ${y}$ are sufficiently small.

We have the famous Baker-Campbell-Hausdoff-Dynkin formula:

Theorem 1 (BCH formula) Let ${G}$ be a finite-dimensional Lie group over the reals with Lie algebra ${{\mathfrak g}}$. Let ${\log}$ be a local inverse of the exponential map ${\exp: {\mathfrak g} \rightarrow G}$, defined in a neighbourhood of the identity. Then for sufficiently small ${x, y \in {\mathfrak g}}$, one has

$\displaystyle \log( \exp(x) \exp(y) ) = R_y(x) = L_x(y).$

See for instance these notes of mine for a proof of this formula (it is for ${R_y}$, but one easily obtains a similar proof for ${L_x}$).

In particular, one can give a neighbourhood of the identity in ${{\mathfrak g}}$ the structure of a local Lie group by defining the group operation ${\ast}$ as

$\displaystyle x \ast y := R_y(x) = L_x(y) \ \ \ \ \ (3)$

for sufficiently small ${x, y}$, and the inverse operation by ${x^{-1} := -x}$ (one easily verifies that ${R_x(-x) = L_x(-x) = 0}$ for all small ${x}$).

It is tempting to reverse the BCH formula and conclude (the local form of) Lie’s third theorem, that every finite-dimensional Lie algebra is isomorphic to the Lie algebra of some local Lie group, by using (3) to define a smooth local group structure on a neighbourhood of the identity. (See this previous post for a definition of a local Lie group.) The main difficulty in doing so is in verifying that the definition (3) is well-defined (i.e. that ${R_y(x)}$ is always equal to ${L_x(y)}$) and locally associative. The well-definedness issue can be trivially disposed of by using just one of the expressions ${R_y(x)}$ or ${L_x(y)}$ as the definition of ${\ast}$ (though, as we shall see, it will be very convenient to use both of them simultaneously). However, the associativity is not obvious at all.

With the assistance of Ado’s theorem, which places ${{\mathfrak g}}$ inside the general linear Lie algebra ${\mathfrak{gl}_n({\bf R})}$ for some ${n}$, one can deduce both the well-definedness and associativity of (3) from the Baker-Campbell-Hausdorff formula for ${\mathfrak{gl}_n({\bf R})}$. However, Ado’s theorem is rather difficult to prove (see for instance this previous blog post for a proof), and it is natural to ask whether there is a way to establish these facts without Ado’s theorem.

After playing around with this for some time, I managed to extract a direct proof of well-definedness and local associativity of (3), giving a proof of Lie’s third theorem independent of Ado’s theorem. This is not a new result by any means, (indeed, the original proofs of Lie and Cartan of Lie’s third theorem did not use Ado’s theorem), but I found it an instructive exercise to work out the details, and so I am putting it up on this blog in case anyone else is interested (and also because I want to be able to find the argument again if I ever need it in the future).

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