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Reading this book, I came across the following formula

$$e^A e^B = e^{A+B}e^{\frac{1}{2}[A,B]}$$

where $A$ and $B$ are two matrices and $[A,B] := AB-BA$.

I tried to find a proof, without success. It is impossible to use the binomial theorem since $A$ and $B$ do not commute. I've thought about developping the product in a power series, but I'm not sure the Cauchy product is allowed when $[A,B] \ne 0$.

I thought that $[A,B^k]$ could be useful so I searched the general expression

$$[A,B^n] = \sum_{i=0}^{n-1} B^i [A,B] B^{n-i-1}$$

Does anyone know a proof of the formula above? If so, is it possible to provide some hints? Or a full proof?

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3 Answers 3

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The equality is true when $A,B$ are quasi-commutative, that is, when $[A,B]$ commutes with both $A$ and $B$.

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The author probably means an approximation rather than a true equality, as $(A+B)+\frac12[A,B]$ are only the first two terms in the BCH formula Zassenhaus formula.

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  • $\begingroup$ How can one be sure that the terms after can be neglected ? $\endgroup$
    – ChocoPouce
    Commented Dec 2, 2013 at 17:24
  • $\begingroup$ @ChocoPouce The trailing terms are not always negligible. You may search the internet for convergence results. For example, see sec. 3 and fig. 1 of Casas et al., Efficient computation of the Zassenhaus formula. $\endgroup$
    – user1551
    Commented Dec 2, 2013 at 18:50
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The book you referenced was "Quantum Physics in One Dimension". The operators in quantum mechanics have to obey particular commutation relations. For instance the operators $x$ and $p$ obey the canonical commutation relation,

$$ \left[ x, p \right] = i I. $$

This allows us to simplify the Zassenhaus formula. The operator $[x,p]$ commutes with every operator because it is proportional to the identity; if you consider the higher order terms in the Zassenhaus formula you will see they involve commutators which must be zero.

Meaning that,

$$\exp(x+p)= \exp(x)\exp(p)\exp(-[x,p]/2),$$

is an exact relationship for the operators $x$ and $p$.

Knowledge of specific commutation relations can allow you to simplify the Zassenhaus formula significantly.

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