I was wondering if a variation of the bin packing problem where the 'size' of a bin is calculated as the product of item sizes in a bin instead of their sum is NP-hard. It seems like it must be, but I don't know how to prove it.
More formally, the problem is as follows:
Given a list of positive integer item sizes $a_1, a_2, ..., a_n$ and a positive integer $x$, find the smallest positive integer $k$ such that it is possible to partition the items into $k$ disjoint sets $S_1, S_2, ..., S_k$ such that the product of sizes in each $S_i$ is $\leq x$.