Integer factorization and Riemann's hypothesis: Why two-item joint replenishment is hard

TL;DR

This paper links the computational difficulty of two-item joint replenishment problems in inventory management to the complexity of integer factorization, showing it is NP-complete under Riemann's Hypothesis, thus explaining why efficient solutions are hard to find.

Contribution

It introduces a novel connection between supply chain optimization and number theory, specifically integer factorization, and proves the problem's NP-completeness assuming Riemann's Hypothesis.

Findings

Two-item joint replenishment is as hard as integer factorization.

Under Riemann's Hypothesis, the problem is NP-complete.

Quantum computers may not efficiently solve this problem.

Abstract

Distribution networks with periodically repeating events often hold great promise to exploit economies of scale. Joint replenishment problems are a fundamental model in inventory management, manufacturing, and logistics that capture these effects. However, finding an efficient algorithm that optimally solves these models, or showing that none may exist, has long been open, regardless of whether empty joint orders are possible or not. In either case, we show that finding optimal solutions to joint replenishment instances with just two products is at least as difficult as integer factorization. To the best of the authors' knowledge, this is the first time that integer factorization is used to explain the computational hardness of any kind of optimization problem. Under the assumption that Riemann's Hypothesis is correct, we can actually prove that the two-item joint replenishment problem with possibly empty joint ordering points is NP-complete under randomized reductions, which implies that not even quantum computers may be able to solve it efficiently. By relating the computational complexity of joint replenishment to cryptography, prime decomposition, and other aspects of prime numbers, a similar approach may help to establish (integer factorization) hardness of additional open periodic problems in supply chain management and beyond, whose solution has eluded standard methods.