We study a two-level uncapacitated lot-sizing problem with inventory bounds that occurs in a supply chain composed of a supplier and a retailer. The first level with the demands is the retailer level and the second one is the supplier level. The aim is to minimize the cost of the supply chain so as to satisfy the demands when the quantity of item that can be held in inventory at each period is limited. The inventory bounds can be imposed at the retailer level, at the supplier level or at both levels. We propose a polynomial dynamic programming algorithm to solve this problem when the inventory bounds are set on the retailer level. When the inventory bounds are set on the supplier level, we show that the problem is NP-hard. We give a pseudo-polynomial algorithm which solves this problem when there are inventory bounds on both levels. In the case where demand lot-splitting is not allowed, i.e. each demand has to be satisfied by a single order, we prove that the uncapacitated lot-sizing problem with inventory bounds is strongly NP-hard. This implies that the two-level lot-sizing problems with inventory bounds are also strongly NP-hard when demand lot-splitting is considered.
Literature reviewFor many practical applications, it is unreasonable to suppose that the inventory capacity is unlimited. In particular, the products that need temperature control or special storage facilities may have a limited storage capacity. This is for example the case in the pharmaceutical industry [1]. These constraints have led to the study of lot-sizing problems with inventory bounds.The single level Uncapacitated Lot-Sizing problem with Inventory Bounds (ULS-IB) was first introduced by Love [2]. He proves that the problem with piecewise concave ordering and holding costs and backlogging can be solved using an O(T 3 ) dynamic programming algorithm. Atamtürk and Küçükyavuz [1] study the ULS-IB Two-level lot-sizing with inventory bounds S-L Phouratsamay, S. Kedad-Sidhoum, F. Pascual problem under the cost structure assumed in Love's paper [2], considering in addition a fixed holding cost. They propose an O(T 2 ) algorithm to solve the problem. They also make a polyhedral study of the ULS-IB problem [3] by considering two cost structures: linear holding costs, linear and fixed holding costs. They provide an exact separation algorithm for each problem. Toczylowski [4] addresses this problem by solving a shortest path problem in O(T 2 ) time. More recently, Hwang and van den Heuvel [5] propose an O(T 2 ) dynamic programming algorithm to solve the ULS-IB problem with backlogging and a concave cost structure by exploiting the so-called Monge property. Gutiérrez et al. [6] improved the time complexity by developing an algorithm that runs in O(TlogT) using the geometric technique of Wagelmans and van Hoesel [7]. However, van den Heuvel et al. [8] show that their algorithm does not provide an optimal solution for the ULS-IB problem. Liu [9] proposes an O(T 2 ) algorithm based on the geometric approach in [7] but Önal et al. [10] prove that his ...