What? MMM is learning Combinatorics!?
Looks like she is playing with the bell sequence now:
bell[n] = number of ways to partition of the set {1, 2, 3, ..., n}
e.g. n = 3:
{1, 2, 3}
{1} {2 3}
{1, 2} {3}
{1, 3} {2}
{1} {2} {3}
so, bell[3] = 5
MMM wonders how to compute the bell sequence efficiently.