A mass split is an operation on a multiset K of positive integers. One mass split handles every element of K at the same time. An element k that is prime is left alone. An element k that is not prime is replaced by the two numbers d and k/d, where d is a divisor of k with 1<d<k. Every divisor that meets the condition is equally likely, and each element draws its divisor independently.
Take K={2,10,12,12} as an example. 2 is prime and stays. The only divisors of 10 strictly between 1 and 10 are 2 and 5, so 10 always becomes {2,5}. Each 12 picks one of 2, 3, 4, 6 with probability 1/4, which turns it into {2,6} or {3,4} with probability 1/2 each. The first mass split therefore gives {2,2,3,3,4,4,5} with probability 0.25, {2,2,2,3,4,5,6} with probability 0.5, and {2,2,2,2,5,6,6} with probability 0.25. A second mass split applied to the last of those gives {2,2,2,2,2,2,3,3,5} and nothing else.
Start from the multiset {N}, which holds the single element N, and repeat the mass split until every element is prime. Compute the expected number of mass splits.