You are given a rectangular $n_1\times n_2$ grid with one light bulb $b_i$ at every node. Each bulb is on or off with probability $p$ and $1-p$, respectively, and furthermore you know that exactly $m$ bulbs are on. Equip the grid with the Manhattan metric $d$, and look at the set of distances between distinct bulbs which are on:
$S:=\{d(b_i,b_j): i\neq j, b_i \textrm{ and } b_j \textrm{ are on}\}.$
To understand how close two lit bulbs should be, I would like to compute the expected value of the random variable $X:=\min S$ (in terms of $n_1,n_2,p$ and $m$). Any hints on how to start thinking about this problem?
Thank you.