Another index notation question hopefully pretty easy

[AxiomOfChoice]

How do I make sense of the index notation expression

<br /> r_j r_i p_j<br />

?

What I really *want* it to be is

<br /> \vec{r} (\vec{r} \cdot \vec{p})<br />

And it turns into this, as long as I can commute the r_i[/tex] and r_j...right?<br /> <br /> (By the way, here \vec{r} is just the position vector and \vec{p} is the momentum vector.)

 

[HallsofIvy]

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

 

[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!

 

[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 :(