Kth-nearest-neighbour distance in a homogeneous Poisson point process
= Kth-nearest-neighbour distance in a homogeneous Poisson point process
For intensity $\lambda$ in $\mathbb R^d$, let $R_k$ be the distance to the $k$th closest point and put $\rho=\lambda v_d$. Then $R_k^d$ is the $k$th arrival time of a rate-$\rho$ Poisson process, so
$$
f_{R_k}(r)=\frac{d\rho^k}{(k-1)!}\,
r^{dk-1}e^{-\rho r^d},
\qquad r>0.
$$