# Another index notation question hopefully pretty easy

1. Mar 20, 2009

### AxiomOfChoice

How do I make sense of the index notation expression

$$r_j r_i p_j$$

?

What I really *want* it to be is

$$\vec{r} (\vec{r} \cdot \vec{p})$$

And it turns into this, as long as I can commute the $r_i[/tex] and [itex]r_j$...right?

(By the way, here $\vec{r}$ is just the position vector and $\vec{p}$ is the momentum vector.)

2. Mar 20, 2009

### HallsofIvy

Staff Emeritus
Well, those are simply coordinates, i.e. numbers, so they are commutative, aren't they?

3. Mar 20, 2009

### confinement

Yes, you can commute them just fine. I know how you feel, after doing a lot of quantum operator calculations it feels like such a breeze to have the commutative property!

4. Mar 21, 2009

### AxiomOfChoice

Thanks guys! Yes, this is actually something that's come up in trying to solve a quantum problem. And yeah...I've done enough of these to where I pretty much don't believe anything commutes anymore :(