Algorithms for Secretary Problems on Graphs and Hypergraphs

We examine several online matching problems, with applications to Internet advertising reservation systems. Consider an edge-weighted bipartite graph G, with partite sets L, R. We develop an 8-competitive algorithm for the following secretary problem: Initially given R, and the size of L, the algorithm receives the vertices of L sequentially, in a random order. When a vertex l \in L is seen, all edges incident to l are revealed, together with their weights. The algorithm must immediately either match l to an available vertex of R, or decide that l will remain unmatched. Dimitrov and Plaxton show a 16-competitive algorithm for the transversal matroid secretary problem, which is the special case with weights on vertices, not edges. (Equivalently, one may assume that for each l \in L, the weights on all edges incident to l are identical.) We use a similar algorithm, but simplify and improve the analysis to obtain a better competitive ratio for the more general problem. Perhaps of more interest is the fact that our analysis is easily extended to obtain competitive algorithms for similar problems, such as to find disjoint sets of edges in hypergraphs where edges arrive online. We also introduce secretary problems with adversarially chosen groups. Finally, we give a 2e-competitive algorithm for the secretary problem on graphic matroids, where, with edges appearing online, the goal is to find a maximum-weight acyclic subgraph of a given graph.Comment: 15 pages, 2 figure

Whole-Page Optimization and Submodular Welfare Maximization with Online Bidders

In the context of online ad serving, display ads may appear on different types of webpages, where each page includes several ad slots and therefore multiple ads can be shown on each page. The set of ads that can be assigned to ad slots of the same page needs to satisfy various prespecified constraints including exclusion constraints, diversity constraints, and the like. Upon arrival of a user, the ad serving system needs to allocate a set of ads to the current webpage respecting these per-page allocation constraints. Previous slot-based settings ignore the important concept of a page and may lead to highly suboptimal results in general. In this article, motivated by these applications in display advertising and inspired by the submodular welfare maximization problem with online bidders, we study a general class of page-based ad allocation problems, present the first (tight) constant-factor approximation algorithms for these problems, and confirm the performance of our algorithms experimentally on real-world datasets. A key technical ingredient of our results is a novel primal-dual analysis for handling free disposal, which updates dual variables using a “level function” instead of a single level and unifies with previous analyses of related problems. This new analysis method allows us to handle arbitrarily complicated allocation constraints for each page. Our main result is an algorithm that achieves a 1 &minus frac 1 e &minus o(1)-competitive ratio. Moreover, our experiments on real-world datasets show significant improvements of our page-based algorithms compared to the slot-based algorithms. Finally, we observe that our problem is closely related to the submodular welfare maximization (SWM) problem. In particular, we introduce a variant of the SWM problem with online bidders and show how to solve this problem using our algorithm for whole-page optimization.postprin

Optimal delivery in display advertising

In display advertising, a publisher targets a specific audience by displaying ads on content web pages. Because the publisher has little control over the supply of display opportunities, the actual supply of ads that it can sell is stochastic. We consider the problem of optimal ad delivery, where an advertiser requests a certain number of impressions to be displayed by the publisher over a certain time horizon. Time is divided into periods, and in the beginning of each period the publisher chooses a fraction of the still unrealized supply to allocate towards fulfilling the advertiser's demand. The goal is to be able to fulfill the demand at the end of the horizon with minimal costs incurred from penalties associated with shortage or over-delivery of ads. We describe optimal policies that are both simple in structure and easy to implement for several variations of this problem

Pass-through of unfair trading practices in EU food supply chains

This report presents the results of the research project “Pass-Through of Unfair Trading Practices in EU Food Supply Chains: Methodology and Empirical Application”. The purpose of the project is to design and test a monitoring system of unfair trading practices (UTP) along the agri-food supply chain. The investigation has special focus on assessment of the “pass-through effect”, defined as the consequences for the entire supply chain of UTPs adopted in a specific transaction. The report includes: (i) a review of the economic literature for a better understanding of the economic principles of UTPs; (ii) a review of available data sources and past experiences in UTP monitoring; (iii) the illustration of two alternative approaches for UTP monitoring: B-SEA (broad-scope empirical analysis) and IDEA (in-depth analysis); (iv) a test application of the two approaches to the EU fresh fruit sector; (v) a comparative analysis of the IDEA and B-SEA results and (vi) a discussion of the implications of our research.JRC.D.4-Economics of Agricultur

Planning and Scheduling Optimization

Although planning and scheduling optimization have been explored in the literature for many years now, it still remains a hot topic in the current scientific research. The changing market trends, globalization, technical and technological progress, and sustainability considerations make it necessary to deal with new optimization challenges in modern manufacturing, engineering, and healthcare systems. This book provides an overview of the recent advances in different areas connected with operations research models and other applications of intelligent computing techniques used for planning and scheduling optimization. The wide range of theoretical and practical research findings reported in this book confirms that the planning and scheduling problem is a complex issue that is present in different industrial sectors and organizations and opens promising and dynamic perspectives of research and development

Optimal delivery in display advertising

In display advertising, a publisher targets a specific audience by displaying ads on content web pages. Because the publisher has little control over the supply of display opportunities, the actual supply of ads that it can sell is stochastic. We consider the problem of optimal ad delivery, where an advertiser requests a certain number of impressions to be displayed by the publisher over a certain time horizon. Time is divided into periods, and in the beginning of each period the publisher chooses a fraction of the still unrealized supply to allocate towards fulfilling the advertiser's demand. The goal is to be able to fulfill the demand at the end of the horizon with minimal costs incurred from penalties associated with shortage or over-delivery of ads. We describe optimal policies that are both simple in structure and easy to implement for several variations of this problem

Whole-page Optimization and Submodular Welfare Maximization with Online Bidders

In the context of online ad serving, display ads may appear on different types of web-pages, where each page includes several ad slots and therefore multiple ads can be shown on each page. The set of ads that can be assigned to ad slots of the same page needs to satisfy various pre-specified constraints including exclusion constraints, diversity constraints, and the like. Upon arrival of a user, the ad serving system needs to allocate a set of ads to the current web-page respecting these per-page allocation constraints. Previous slot-based settings ignore the important concept of a page, and may lead to highly suboptimal results in general. In this paper, motivated by these applications in display advertising and inspired by the submodular welfare maximization problem with online bidders, we study a general class of page-based ad allocation problems, present the first (tight) constant-factor approximation algorithms for these problems, and confirm the performance of our algorithms experimentally on real-world data sets. A key technical ingredient of our results is a novel primal-dual analysis for handling free-disposal, which updates dual variables using a "level function" instead of a single level, and unifies with previous analyses of related problems. This new analysis method allows us to handle arbitrarily complicated allocation constraints for each page. Our main result is an algorithm that achieves a 1 − 1 e − o(1) competitive ratio. Moreover, our experiments on real-world data sets show significant improvements of our page-based algorithms compared to the slot-based algorithms. Finally, we observe that our problem is closely related to the submodular welfare maximization (SWM) problem. In particular, we introduce a variant of the SWM problem with online bidders, and show how to solve this problem using our algorithm for whole page optimization

Linking middle-chain actors to the environmental impacts of food producers and consumers: Underlying drivers and policy implications

The majority of GHG emissions, pollution and biodiversity loss from food systems come from agricultural and dietary practices - that is to say, what is produced on the farm (and eventually eaten) and how we produce it. However, whilst farmers and consumers produce the majority of these impacts, they are not necessarily wholly in control of or always have the ability to mitigate the impacts they produce. Rather, farmers and consumers are operating and living in complex networks where relationships with other actors in the system drive certain behaviours while limiting others. Because of their size and buying power, global food manufacturers and retailers in particular have significant weight in these networks of relationships. A rapid review of the academic literature examining the interface between middle-chain actors and farmers and consumers was conducted. This evidence synthesis showed that, while the drivers of agricultural and dietary practices are multi-faceted and complex, middle-chain actors, particularly food manufacturers and retailers, exert pressures on farmers and consumers that result in behaviours which lead to negative environmental impacts. In farming, there is strong evidence that, in horticulture, competition between farmers for a limited number of unpredictable retailer contracts in which stringent cosmetic standards are applied leads to over-production and on-farm food loss. In arable and staple food production, emerging evidence suggests the food processing industry has a role in driving on-farm fertiliser use and driving biodiversity loss through the production of ultra-processed foods. These practices are enabled through the mobilisation of middlechain actors’ power in contract negotiations, where retailers and manufacturers are able to off-load the economic and environmental costs of food loss and environmental damage to farmers. This leaves no incentive for these middle-chain actors to measure, report on and adapt their processes to reduce environmental impacts elsewhere in their value chain. In households, manufacturers and retailers employ a range of tactics to increase sales that drive over-purchasing and consumption of foods at a household and individual level. Interestingly, the evidence is mixed on the impact of some of the more seemingly obvious drivers. It is unclear whether advertising ultra-processed foods (UPFs) to adults drives over-consumption. It is also unclear whether price promotions drive household food waste. Indeed, evidence is thin that processors and retailers are major drivers of household food waste. However, it is clear that promotional strategies such as pack and portion sizes and in-store product placement, underpinned by an ‘omnipresence’ strategy, play a major role in the amount purchased and consumed, particularly for ultra-processed foods. The findings of this review point to the importance of taking a policy approach that will facilitate a transition to a value chain of mutual responsibility, where actors along value chains must internalise the risk and costs of Scope III environmental impacts as much as Scope I. This report points to several concrete policy actions that must be taken to ensure that efforts across the value chain are effectively implemented and transparently monitored. Such advances in the EU legislative framework for sustainable food systems will go a long way in advancing value chains towards mutual responsibility for transitioning to a more equitable and sustainable food system

Optimal delivery in display advertising

Abstract: In online display advertising, a publisher targets a specific audience by displaying ads on content web pages. Because the publisher has little control over internet traffic, the supply of display opportunities is stochastic. I consider the problem of optimal ad delivery, where advertisers request a number of ads to be displayed by the publisher over a certain time horizon. Time is discrete and divided into periods. In the beginning of each period the publisher chooses fractions of the still unrealized supply to allocate towards fulfilling the advertisers' demands. The goal is to be able to fulfill the demand at the end of the horizon with minimum costs incurred from penalties associated with shortage or overdelivery of ads as well as advertiser-specific delivery constraints. I derive optimal policies that are both simple in structure and easy to implement for several variations of this problem