# What is the probability that you roll a dice with s sides for n times and t sides appeared once?

You roll a dice with $s$ sides for $n$ times. What is the probability that $t$ sides appeared once?

Example with $s=2$, $n=3$: you roll a dice with 2 sides for 3 times. possible outcomes: '1' and '2'.

• $p(t=0)$ is the probability that '1' appeared 0 times and '2' appeared 3 times + the probability that '2' appeared 0 times and '1' appeared 3 times, that is 1/4.
• $p(t=1)$ is the probability that '1' appeared once and '2' appeared twice + the probability that '2' appeared once and '1' appeared twice, that is 3/4.
• $p(t=2)=0$, because it is impossible that both '1' and '2' appear once.

If there is a formula for $p(s,n,t)$ it will be awesome, but also an algorithm is welcome.

The $t$ singleton sides (sides which appear once) can be chosen as an ordered sequence in $t! {s\choose t}$ ways; this sequence can appear among the $n$ rolls in ${n\choose t}$ ways and the remaining $n-t$ rolls constitute an ordered partition of $n-t$ elements into $s-t-k$ classes, $k=0,1,\ldots s-t-1$, no class having fewer than two elements, which can be chosen in $\sum_{k=0}^{s-t-1}(s-t-k)!{{s-t}\choose k}\begin{Bmatrix} n-t\\ s-t-k \end{Bmatrix}_2$ ways. (The integer $k$ counts the number of classes that do not appear at all.) Multiply these together and divide by $s^n$, the number of $n$ sequences, to obtain the desired probability
$$p=\frac{s!}{s^n} {n\choose t}\sum_{k=0}^{s-t-1}\frac{1}{k!}\begin{Bmatrix} n-t\\ s-t-k \end{Bmatrix}_2$$
The generating function for the coefficients $\begin{Bmatrix} n\\ k \end{Bmatrix}_2$ is $$\sum_{n\geq 0}\begin{Bmatrix} n\\ k \end{Bmatrix}_2\frac{x^n}{n!}=\frac{1}{k!}\left(e^x-1-x\right)^k$$
• it's explained on page 12 in the cited paper, where they define this generalized Stirling coefficient with a subscript $h$ as the number of unordered partitions of an $n$-set into $k$ classes of at least $h$ elements each; here I need $h=2$. Commented Jun 10, 2017 at 12:58