Fredrik Olsson

An inventory model with unidirectional lateral transshipments

This paper deals with a continuous review inventory system with Poisson demand, in which lateral transshipments are allowed. In case of a shortage at a location, another location acts as a supplier, if it is possible. A common assumption in earlier papers is that transshipments are allowed between all locations. This network configuration may, however, not be the best choice for many reasons. One such reason is that it may be difficult to establish contracts between locations regarding the design of the transshipment policy. Another reason is that a system with many transshipment links is much more complex than a system with few transshipment links. In this paper, we study a system where transshipments are allowed only in one direction. This may be a reasonable policy if the locations have very different backorder/lost sales costs. Our approach is relatively simple and fast, and works well in most cases.

Inventory problems with perishable items: Fixed lifetimes and backlogging

Our model deals with a single-product and a single-stock location with Poisson demand. The replenishment leadtime from the external supplier is fixed. The lifetime of the product is also fixed, and aging is assumed to begin when the order is placed. When the age of a unit has reached its lifetime, the unit is useless and thus discarded from the system. The replenishment policy is assumed to be an order-up-to S-policy. Demand that cannot be met immediately is backordered. We consider three different cases where the service requirements are represented by: (1) backorder costs per unit, (2) a service level constraint, (3) backorder costs per unit and time unit. Cases 1 and 2 are solved exactly, while an approximation is developed for case 3. We show how the results from an earlier paper assuming lost sales can be used to solve the considered problems. Our results are compared to the results in a related paper considering (Q, r)-policies.

Probing the stability of the modular family 10 xylanase from Rhodothermus marinus

The thermophilic bacterium Rhodothermus marinus produces a modular xylanase (Xyn10A) consisting of two N-terminal carbohydrate-binding modules (CBMs), followed by a domain of unknown function, and a catalytic module flanked by a fifth domain. Both Xyn10A CBMs bind calcium ions, and this study explores the effect of these ions on the stability of the full-length enzyme. Xyn10A and truncated forms thereof were produced and their thermostabilities were evaluated under different calcium loads. Studies performed using differential scanning calorimetry showed that the unfolding temperature of the Xyn10A was significantly dependent on the presence of Ca2+, and that the third domain of the enzyme binds at least one Ca2+. Thermal inactivation studies confirmed the role of tightly bound Ca2+ in stabilizing the enzyme, but showed that the presence of a large excess of this ion results in reduced kinetic stability. The truncated forms of Xyn10A were less stable than the full-length enzyme, indicative of module/domain thermostabilizing interactions. Finally, possible roles of the two domains of unknown function are discussed in the light of this study. This is the first report on the thermostabilizing role of calcium on a modular familyxa010 xylanase that displays multiple calcium binding in three of its five domains/modules.

Emergency Lateral Transshipments in a Two-Location Inventory System with Positive Transshipment Leadtimes

We consider a single-echelon continuous review inventory system for spare parts with two parallel locations. Each location faces independent Poisson demand and backorders are allowed. In this paper we consider the possibility of lateral transshipments between the locations. The transshipment leadtime is positive and deterministic, and there is an additional cost for making a transshipment. We suggest a transshipment policy which is based on the timing of all outstanding orders, and develop and solve a heuristic model by using theory and concepts from doubly stochastic Poisson processes and also partial differential equations. A simulation study indicates that our heuristic works very well, and that the relative cost increase of disregarding the transshipment leadtime may be quite high. Our results also indicate that it is, in general, worth the effort of reducing the transshipment leadtime, even if it is already relatively short.

Analysis of inventory policies for perishable items with fixed leadtimes and lifetimes

This paper deals with a continuous review inventory system with perishable items and Poisson demand. Lifetimes and leadtimes are assumed to be fixed. First, a quite general base-stock model is developed where a number of combinations between backorder- and lost sales policies are evaluated and optimized. The solution technique for all these combinations is exact. Secondly, we consider the case with non-negligible ordering costs and assume that the inventory policy employed is the commonly used (

Age-based inventory control in a multi-echelon system with emergency replenishments

R,Q

Modelling two-echelon serial inventory systems with perishable items

R,Q) policy. We develop a new heuristic approach for evaluating and analyzing the proposed