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## Homework Statement

Show that the inner product of the Pauli matrices, σ, and the momentum operator, [itex]\vec{p}[/itex], is given by:

σ [itex]\cdot[/itex] [itex]\vec{p}[/itex] = [itex]\frac{1}{r^{2}}[/itex] (σ [itex]\cdot[/itex] [itex]\vec{r}[/itex] )([itex]\frac{\hbar}{i}[/itex] r [itex]\frac{\partial}{\partial r}[/itex] + iσ [itex]\cdot[/itex] [itex]\vec{L}[/itex]),

where [itex]\vec{L}[/itex] is the angular momentum operator and [itex]\vec{r}[/itex] is the displacement vector.

## Homework Equations

p[itex]_{x}[/itex] = [itex]\frac{\hbar}{i}[/itex] [itex]\frac{\partial}{\partial x}[/itex]

[itex]\vec{L}[/itex] = [itex]\vec{r}[/itex] × [itex]\vec{p}[/itex]

## The Attempt at a Solution

I figured that I could write:

[itex]\vec{p}[/itex] = [itex]\frac{\hbar}{i}[/itex] [itex]\frac{\partial}{\partial r}[/itex] [itex]\hat{r}[/itex]

So then:

σ [itex]\cdot[/itex] [itex]\vec{p}[/itex] = (σ [itex]\cdot[/itex] [itex]\hat{r}[/itex]) [itex]\frac{\hbar}{i}[/itex] [itex]\frac{\partial}{\partial r}[/itex]

= [itex]\frac{1}{r}[/itex] (σ [itex]\cdot[/itex] [itex]\vec{r}[/itex]) [itex]\frac{\partial}{\partial r}[/itex]

... But that clearly gets me nowhere. Help?