= Two-particle number density
{title2=$n_2(\mathbf x,\mathbf x_\star,t)$}
The spatial marginal $n_2=\int f_2\,d^3v\,d^3v_\star$ of a number-normalized <two-particle distribution function>. It counts ordered distinct pairs, so its integral over both positions is $N(N-1)$. Exchange symmetry gives $n_2(\mathbf x,\mathbf x_\star)=n_2(\mathbf x_\star,\mathbf x)$. In a homogeneous isotropic fluid it is $n^2g(|\mathbf x_\star-\mathbf x|)$, where $g$ is the <radial distribution function>.
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