Special-case closed form of the Baker-Campbell-Hausdorff formula

Abstract

The Baker-Campbell-Hausdorff formula is a general result for the quantity $Z(X,Y)=\ln (e^X{e}^Y)$, where $X$ and $Y$ are not necessarily commuting. For completely general commutation relations between $X$ and $Y$, (the free Lie algebra), the general result is somewhat unwieldy. However in specific physics applications the commutator $[X,Y]$, while non-zero, might often be relatively simple, which sometimes leads to explicit closed form results. We consider the special case $[X,Y]=uX+vY+cI$, and show that in this case the general result reduces to \[ Z(X,Y)=\ln( e^X e^Y ) = X+Y+ f(u,v) \; [X,Y]. \] Furthermore we explicitly evaluate the symmetric function $f(u,v)=f(v,u)$, demonstrating that \[ f(u,v) = {(u-v)e^{u+v}-(ue^u-ve^v)\over u v (e^u - e^v)}, \] and relate this to previously known results. For instance this result includes, but is considerably more general than, results obtained from either the Heisenberg commutator $[P,Q]=-i\hslash I$ or the creation-destruction commutator $[a,a^{†}]=I$.

V1: 5 pages. V2: 4 references added, some minor typos fixed, some discussion added. No change in conclusions. Now 6 pages. This version accepted for publication in Journal of Physics A: Mathematical and Theoretical

References in corpus (1)

• An efficient algorithm for computing the Baker-Campbell-Hausdorff series and some of its applications

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