Can't be more precise than the title of my question!
How many possibilities to put N balls in n bags with the additional constraint that each bag can contain no more than $r_i$ ($r_1$, $r_2$, ... $r_n$) balls?
I saw this question at different places and I also read a nice answer here.
I'd love to ask the person who answered to comment a little bit on his last formula (what are $e_i$ and $c_i$?)! Unfortunately, I am a newbie on this forum so I can't comment, I can only post questions!
So if someone can help me, comment a little bit, it would be nice!