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What is the commutator of the exponential derivative operator and the exponential position operator? \begin{align} \left[\exp(\partial_x),\exp(x)\right] =\exp(\partial_x)\exp(x) -\exp(x)\exp(\partial_x)=~? \end{align}

I first wrote down the exponentials into there power series sum \begin{align} \left[\exp(\partial_x),\exp(x)\right] = \sum_{n=0}^{\infty} \dfrac{\partial_x^n}{n!} \sum_{m=0}^{\infty} \dfrac{\partial_x^m}{m!} - \sum_{n=0}^{\infty} \dfrac{x^n}{n!} \sum_{m=0}^{\infty} \dfrac{\partial_x^m}{m!}, \end{align} and then grouped like terms \begin{align} \left[\exp(\partial_x),\exp(x)\right] = \sum_{n=0}^{\infty} \sum_{m=0}^{\infty} \dfrac{ \left[ \partial_x^n x^m - x^n \partial_x^m \right] }{n!m!}, \end{align} and then evaluate the derivatives \begin{align} \left[\exp(\partial_x),\exp(x)\right] = \sum_{n=0}^{\infty} \sum_{m=0}^{\infty} \dfrac{ \left[ \partial_x^n (x^m) + x^m \partial_x^n - x^n \partial_x^m \right] }{n!m!}. \end{align} I am not sure if$$ \partial_x^n\left(x^m\right)~=~\frac{m!}{\left(m-n+1\right)!} \, x^{m-n} \,,$$and then I got stuck.

Qmechanic
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2 Answers2

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Suresh is right, the Baker-Campbell-Hausdorff formula will save you. If $\left[A,B\right]$ commutes with $A$ and $B$ you have,

$$ \exp\left(A\right)\exp\left(B\right) = \exp\left(A+B+\frac{1}{2}\left[A,B\right]\right) \, .$$

Then the answer to your question is

$$ \exp\left(\partial_x\right)\exp\left(x\right) = 2 \exp\left(x+\partial_x\right) \sinh(1/2) \, .$$

DanielSank
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Besides the truncated BCH formula (which is also proven in this Phys.SE post and mentioned in Steven Mathey's answer), in practice one often wants to normal order the differential operators, i.e. putting all the $x$'s to the left of all the $\partial_x$. To this end the formula

$$\tag{1} e^{a\partial_x}f(x) ~=~ f(x+a)e^{a\partial_x}. $$

is useful. Hence the sought-for normal-ordered commutator becomes

$$\tag{2} [e^{a\partial_x}, e^{bx}] ~=~(e^{b(x+a)}-e^{bx})e^{a\partial_x}.$$

Qmechanic
  • 220,844