]> $$f\left(k;r,n,N\right)=\frac{n!r!\left(N-n\right)!\left(N-r\right)!}{N!k!\left(n-k\right)!\left(r-k\right)!\left(N-n-r+k\right)!};\max \left(\mathrm{0,}N-r-n+k\right)<=k<=\min \left(r,n\right)$$