# Question about commuting operators

1. Oct 26, 2009

### russdot

If A is an operator, is it correct/allowed to say:
$$Ae^{iA} = e^{iA}A$$

Thanks

2. Oct 26, 2009

### Ben Niehoff

Think of how $e^{A}$ is defined.

3. Oct 26, 2009

### russdot

Ah yes, so since $$e^{A}$$ is defined with a power series $$e^{A} = 1 + A + \frac{A^{2}}{2!} + \frac{A^{3}}{3!} + ...$$ and A commutes with itself then A would commute with $$e^{A}$$
Thanks!