Past exam of the mathematics course of the University of Cambridge / 2016 / ia / Paper 2 / 10F / a / Solution

For any fixed bin, each ball supplies an independent Bernoulli distribution indicator with success probability $1/m$. Summing the $n$ indicator variables givesThe full occupancy vector has a multinomial distribution, the basic uniform balls-in-bins allocation. Distinct bin counts are not independent when $n\ge 1$ and $m\ge 2$. To see this, write $B_i={\sum }_{r=1}^n{I}_{r,i}$. For the same ball, $I_{r,i}{I}_{r,j}=0$ if $i\ne j$, while indicators from distinct balls are independent. Their covariance is thereforeWith no balls the counts are deterministic; with one bin there are no distinct bin pairs. These degenerate cases do not contradict the conclusion.