# Problem understanding operator algebra

1. Oct 27, 2013

### Luna=Luna

"It is left as a problem for the reader to show that if $[S,T]$ commutes with S and T, then $[e^{tT}, S] = -t[S,T]e^{tT}$

I'm not sure if i'm missing something here, but i dont even see how it is possible to arrive at this answer.

I get:
$$[e^{tT}, S] = e^{tT}S - Se^{tT}$$

Then using the fact that $[S,T]$ commutes with S and T this gives:

$SST-STS = STS-TSS$
and
$TST-TTS = STT-TST$

and see no way to go further.
One major thing is I don't even see how the factor of -t just appears in the identity?
$[e^{tT}, S] = -t[S,T]e^{tT}$

2. Oct 27, 2013

### MisterX

The typical way to do something like this would be to use a Taylor expansion of the function of the operator, in this case $e^{tT}$. What you were trying to show is an example of a more general result.