Simplifying the Reinsch algorithm for the Baker-Campbell-Hausdorff series

Abstract

The Baker-Campbell-Hausdorff series computes the quantity \begin{equation*} Z(X,Y)=\ln\left( e^X e^Y \right) = \sum_{n=1}^\infty z_n(X,Y), \end{equation*} where $X$ and $Y$ are not necessarily commuting, in terms of homogeneous multinomials $z_n(X,Y)$ of degree $n$. (This is essentially equivalent to computing the so-called Goldberg coefficients.) The Baker-Campbell-Hausdorff series is a general purpose tool of wide applicability in mathematical physics, quantum physics, and many other fields. The Reinsch algorithm for the truncated series permits one to calculate up to some fixed order $N$ by using $(N+1)\times(N+1)$ matrices. We show how to further simplify the Reinsch algorithm, making implementation (in principle) utterly straightforward. This helps provide a deeper understanding of the Goldberg coefficients and their properties. For instance we establish strict bounds (and some equalities) on the number of non-zero Goldberg coefficients. Unfortunately, we shall see that the number of terms in the multinomial $z_n(X,Y)$ often grows very rapidly (in fact exponentially) with the degree $n$. We also present some closely related results for the symmetric product \begin{equation*} S(X,Y)=\ln\left( e^{X/2} e^Y e^{X/2} \right) = \sum_{n=1}^\infty s_n(X,Y). \end{equation*} Variations on these themes are straightforward. For instance, one can just as easily consider the series \begin{equation*} L(X,Y)=\ln\left( e^{X} e^Y e^{-X} e^{-Y}\right) = \sum_{n=1}^\infty \ell_n(X,Y). \end{equation*} This type of series is of interest, for instance, when considering parallel transport around a closed curve. Several other related series are investigated.