# Probability of at least k distinct events occuring after N trials

Given a finite set $|S|$ of distinct events, each with a $1/|S|$ probability of occurrence, what is the probability that at least k distinct events occur after N trials? For every trial, there is guaranteed to be exactly one event that occurs. Events are repeatable.

For instance, take $S = \{rain, sun, snow\}$. After $N=10$ days, what is the probability that at least $k=2$ distinct types of weather have occurred?

In my specific problem, I have $|S| = 35$, so a method of approximation would be preferred.

Let $m=|S|$. Then the probability that exactly $k$ distinct events have occurred is $$\dfrac{S_2(N,k) \, m! }{ (m-k)!\, m^N}$$ where $S_2(N,k)$ is a Stirling number of the second kind, and the probability that at least $k$ distinct events have occurred is $$\sum_{j=k}^{\min(m,N)} \dfrac{S_2(N,j) \, m! }{ (m-j)!\, m^N}$$
For example, $S_2(10,2)=511$ and $S_2(10,3)=9330$ so in your case of $k=2$, $|S|=m=3$ and $N=10$ you would have $\frac{511 \times 3! }{ 1! \times 3^{10}} + \frac{9330 \times 3! }{ 0! \times 3^{10}} = \frac{59046}{59049} \approx 0.9999492$ though in this particular case it might have been quicker to calculate via the probability of only one distinct event occurring, giving the same $1-3 \times \frac1{3^{10}}$ result