# Tag Info

The impression I got form the equation is that you model your system as being composed of a collection of point particles. The index $i$ indexes this set of point particles (as opposed to indexing the components of a vector). One could create a spatial velocity distribution by saying $\vec{v}(\vec{r})=\sum_i \vec{v}_i \delta(\vec{r} - \vec{r}_i)$, where ...