This question has been asked in different forms but I couldn't find a satisfactory answer (this came close but not close enough).
Assume I have 4 marbles. There are effectively only 5 ways of dividing them into groups of size at least 1 and at most 4:
1) o o o o
2) o o oo
3) o ooo
4) oo oo
4) oooo
I am wondering if there is a generalized formula for this? The key element here is that the groups can but not must be the same size and that the elements are identical (i.e. ooo o and o ooo are identical).