# A few reference questions about the Baker–Campbell–Hausdorff formula

I'm looking at the article Baker–Campbell–Hausdorff formula - Wikipedia and I have a few questions.

Under the "Special cases" section, there is a notation $$\DeclareMathOperator{\ad}{ad}$$

$$\log(e^X e^Y) = X + \frac{\ad_X}{1 - e^{-\ad_X}}Y + O(Y^2),$$

but it doesn't provide a reference for this formula. Who discovered this representation of the BCH formula and does it hold in the general case for general $$X$$ and $$Y$$ or is this only valid in the case $$[X,Y] = sY$$?

Another question related to this, is there a name for when $$[X,Y] = sY$$?

A third question is, what is the proper formal Taylor expansion for the $$\frac{\ad_X}{1-e^{-\ad_X}}$$ term? Is this convergence of this formal Taylor expansion limited by any particular spectral radius? The wiki article does not seem to go into detail about that.

• There is a nice discussion of this, not answering your questions perhaps, on terrytao.wordpress.com/tag/baker-campbell-hausdorff-formula Sep 26 at 15:38
• @BenMcKay, re, there are three articles in that tag; did you mean Tao - The $C^{1, 1}$ Baker–Campbell–Hausdorff formula? Sep 26 at 16:21
• I don't know any name for it, but the situation $[X, Y] = s Y$ is precisely the case when there is a representation of the Lie algebra of the so called “$a x + b$ group” $\left\{\begin{pmatrix} a & b \\ 0 & 1 \end{pmatrix}\right\}$ sending $\begin{pmatrix} 1 & 0 \\ 0 & 0 \end{pmatrix}$ to $X$ and $\begin{pmatrix} 0 & 1 \\ 0 & 0 \end{pmatrix}$ to $Y$ … so perhaps that could be used to make up a name? Sep 26 at 16:24
• See also the 2020 paper "Notes on the theorem of Baker-Campbell-Hausdorff-Dynkin" by Michael Mueger (math.ru.nl/~mueger/PDF/BCHD.pdf). Sep 26 at 18:32
• And w.r.t. numerical integration of diff eqs, see the discussion surrounding eqns. 2.45 and 3.2 on pp. 33 and 38 of "Lie-group methods" by Iserles, Munthe-Kaas, Nørsett, and Zanna (damtp.cam.ac.uk/user/na/NA_papers/NA2000_03.pdf). Sep 26 at 18:56

$$\DeclareMathOperator\ad{ad}$$The identity $$\log(e^X e^Y) = X + \frac{\ad_X}{1 - e^{-\ad_X}}Y + O(Y^2)\tag{*}\label{star}$$ is due to Poincaré [1], who showed that $$W(t)=\log (e^X e^{tY})$$ solves the ODE $$W'(t)=\frac{\ad_{W(t)}}{1-e^{-\ad_{W(t)}}}Y,\;\;W(0)=X.$$ The identity \eqref{star} is the solution to first order in $$t$$. It holds generally, you don't need $$[X,Y]=sY$$.

[1] H. Poincaré, Sur les groupes continus, Trans. Cambridge Phil. Soc., 18, 220–255 (1900). JFM 31.0386.01