On kernels, defaults and even graphs

Extensions in prerequisite-free, disjunction-free default theories have been shown to be in direct correspondence with kernels of directed graphs; hence default theories without odd cycles always have a ``standard'' kind of an extension. We show that, although all ``standard'' extensions can be enumerated explicitly, several other problems remain intractable for such theories: Telling whether a non-standard extension exists, enumerating all extensions, and finding the minimal standard extension. We also present a new graph-theoretic algorithm, based on vertex feedback sets, for enumerating all extensions of a general prerequisite-free, disjunction-free default theory (possibly with odd cycles). The algorithm empirically performs well for quite large theories

A Newsvendor Approach to Compliance and Production under Cap and Trade Emissions Regulation

Since the 1990s, governmental agencies have increasingly turned to market based cap and trade programs to control the emission of pollutants. Firms subject to cap and trade regulation are typically required to acquire emissions allowances via open auction markets. The cost to acquire allowances may impose a substantial financial burden on a firm. While emissions reduction efforts may eliminate some firm\u27s need to acquire additional allowances, there are still numerous firms that need to purchase additional allowances on the open market. This study presents a new forward buying heuristic, designed for those firms that need to purchase emissions allowances via auctions, which reduces the impact of emissions allowance acquisitions on the firms\u27 financial performance. The heuristic, designated as the Newsvendor Production Planning with Emissions Allowance Forward Buying (NPPAFB) method, applies a forward buying algorithm to determine the number of periods for which to forward buy allowances, the current production order up to level, and the current and future emissions allowance requirements (which serves as the order up to level for allowance purchases). Additionally, NPPAFB also authorizes unused emissions allowances to be sold when market conditions are favorable. Compared against three existing production planning and allowance procurement strategies, a simulation exercise finds that the NPPAFB method significantly reduces a firm\u27s emissions allowance expenditures. These results indicate that heuristic can be readily adopted by any firm that is required to procure emissions allowances via open markets in an effort to improve the firm\u27s profitability

A multiple ship routing and speed optimization problem under time, cost and environmental objectives

The purpose of this paper is to investigate a multiple ship routing and speed optimization problem under time, cost and environmental objectives. A branch and price algorithm as well as a constraint programming model are developed that consider (a) fuel consumption as a function of payload, (b) fuel price as an explicit input, (c) freight rate as an input, and (d) in-transit cargo inventory costs. The alternative objective functions are minimum total trip duration, minimum total cost and minimum emissions. Computational experience with the algorithm is reported on a variety of scenarios. © 2017 Elsevier Lt

Dynamic vehicle routing problems: Three decades and counting

Since the late 70s, much research activity has taken place on the class of dynamic vehicle routing problems (DVRP), with the time period after year 2000 witnessing a real explosion in related papers. Our paper sheds more light into work in this area over more than 3 decades by developing a taxonomy of DVRP papers according to 11 criteria. These are (1) type of problem, (2) logistical context, (3) transportation mode, (4) objective function, (5) fleet size, (6) time constraints, (7) vehicle capacity constraints, (8) the ability to reject customers, (9) the nature of the dynamic element, (10) the nature of the stochasticity (if any), and (11) the solution method. We comment on technological vis-à-vis methodological advances for this class of problems and suggest directions for further research. The latter include alternative objective functions, vehicle speed as decision variable, more explicit linkages of methodology to technological advances and analysis of worst case or average case performance of heuristics.© 2015 Wiley Periodicals, Inc