# QM: Identity

1. Apr 12, 2011

### Niles

1. The problem statement, all variables and given/known data
Hi guys

In my book they use the following identity

$$e^{ - i\widehat Ht/\hbar } = e^{ - i\widehat Tt/\hbar } e^{ - i\widehat Vt/\hbar } + O(t^2 )$$

where H = T+V, and the last term means "terms of order t2 or higher". I can't quite see how they reach this identity. First, I know that T and V do not commute, so I guess that is where the O(t2) comes from.

Any help will be appreciated. (BTW, this is not an exercise problem -- just something in my book that I cannot understand).

Best,
Niles.

2. Apr 12, 2011

### vela

Staff Emeritus
Look up the Baker-Hausdorff lemma.