764 research outputs found
Efficient Neural Neighborhood Search for Pickup and Delivery Problems
We present an efficient Neural Neighborhood Search (N2S) approach for pickup
and delivery problems (PDPs). In specific, we design a powerful Synthesis
Attention that allows the vanilla self-attention to synthesize various types of
features regarding a route solution. We also exploit two customized decoders
that automatically learn to perform removal and reinsertion of a
pickup-delivery node pair to tackle the precedence constraint. Additionally, a
diversity enhancement scheme is leveraged to further ameliorate the
performance. Our N2S is generic, and extensive experiments on two canonical PDP
variants show that it can produce state-of-the-art results among existing
neural methods. Moreover, it even outstrips the well-known LKH3 solver on the
more constrained PDP variant. Our implementation for N2S is available online.Comment: Accepted at IJCAI 2022 (short oral
A Polyhedral Study of Mixed 0-1 Set
We consider a variant of the well-known single node fixed charge network flow set with constant capacities. This set arises from the relaxation of more general mixed integer sets such as lot-sizing problems with multiple suppliers. We provide a complete polyhedral characterization of the convex hull of the given set
Integrated Models and Tools for Design and Management of Global Supply Chain
In modern and global supply chain, the increasing trend toward product variety, level of service, short delivery delay and response time to consumers, highlight the importance to set and configure smooth and efficient logistic processes and operations.
In order to comply such purposes the supply chain management (SCM) theory entails a wide set of models, algorithms, procedure, tools and best practices for the design, the management and control of articulated supply chain networks and logistics nodes.
The purpose of this Ph.D. dissertation is going in detail on the principle aspects and concerns of supply chain network and warehousing systems, by proposing and illustrating useful methods, procedures and support-decision tools for the design and management of real instance applications, such those currently face by enterprises.
In particular, after a comprehensive literature review of the principal warehousing issues and entities, the manuscript focuses on design top-down procedure for both less-than-unit-load OPS and unit-load storage systems. For both, decision-support software platforms are illustrated as useful tools to address the optimization of the warehousing performances and efficiency metrics. The development of such interfaces enables to test the effectiveness of the proposed hierarchical top-down procedure with huge real case studies, taken by industry applications.
Whether the large part of the manuscript deals with micro concerns of warehousing nodes, also macro issues and aspects related to the planning, design, and management of the whole supply chain are enquired and discussed.
The integration of macro criticalities, such as the design of the supply chain infrastructure and the placement of the logistic nodes, with micro concerns, such the design of warehousing nodes and the management of material handling, is addressed through the definition of integrated models and procedures, involving the overall supply chain and the whole product life cycle.
A new integrated perspective should be applied in study and planning of global supply chains. Each aspect of the reality influences the others. Each product consumed by a customer tells a story, made by activities, transformations, handling, processes, traveling around the world. Each step of this story accounts costs, time, resources exploitation, labor, waste, pollution. The economical and environmental sustainability of the modern global supply chain is the challenge to face
ProbleĢmes de tourneĢes de veĢhicules avec contraintes de chargement
Cette theĢse sāinteĢresse aux probleĢmes de tourneĢes de veĢhicules ouĢ lāon retrouve des contraintes de chargement ayant un impact sur les seĢquences de livraisons permises. Plus particulieĢrement, les items placeĢs dans lāespace de chargement dāun veĢhicule doivent eĢtre directement accessibles lors de leur livraison sans quāil soit neĢcessaire de deĢplacer dāautres items. Ces probleĢmes sont rencontreĢs dans plusieurs entreprises de transport qui livrent de gros objets (meubles, eĢlectromeĢnagers).
Le premier article de cette theĢse porte sur une meĢthode exacte pour un probleĢme de confection dāune seule tourneĢe ouĢ un veĢhicule, dont lāaire de chargement est diviseĢe en un certain nombre de piles, doit effectuer des cueillettes et des livraisons respectant une contrainte de type dernier entreĢ, premier sorti. Lors dāune collecte, les items recueillis doivent neĢcessairement eĢtre deĢposeĢs sur le dessus de lāune des piles. Par ailleurs, lors dāune livraison, les items doivent neĢcessairement se trouver sur le dessus de lāune des piles. Une meĢthode de seĢparation et eĢvaluation avec plans seĢcants est proposeĢe pour reĢsoudre ce probleĢme.
Le second article preĢsente une meĢthode de reĢsolution exacte, eĢgalement de type seĢparation et eĢvaluation avec plans seĢcants, pour un probleĢme de tourneĢes de veĢhicules avec chargement dāitems rectangulaires en deux dimensions. Lāaire de chargement des veĢhicules correspond aussi aĢ un espace rectangulaire avec une orientation, puisque les items doivent eĢtre chargeĢs et deĢchargeĢs par lāun des coĢteĢs. Une contrainte impose que les items dāun client soient directement accessibles au moment de leur livraison.
Le dernier article aborde une probleĢme de tourneĢes de veĢhicules avec chargement dāitems rectangulaires, mais ouĢ les dimensions de certains items ne sont pas connus avec certitude lors de la planification des tourneĢes. Il est toutefois possible dāassocier une distribution de probabiliteĢs discreĢte sur les dimensions possibles de ces items. Le probleĢme est reĢsolu de manieĢre exacte avec la meĢthode L-Shape en nombres entiers.In this thesis, we study mixed vehicle routing and loading problems where a constraint is imposed on delivery sequences. More precisely, the items in the loading area of a vehicle must be directly accessible, without moving any other item, at delivery time. These problems are often found in the transportation of large objects (furniture, appliances).
The first paper proposes a branch-and-cut algorithm for a variant of the single vehicle pickup and delivery problem, where the loading area of the vehicle is divided into several stacks. When an item is picked up, it must be placed on the top of one of these stacks. Conversely, an item must be on the top of one of these stacks to be delivered. This requirement is called āLast In First Outā or LIFO constraint.
The second paper presents another branch-and-cut algorithm for a vehicle routing and loading problem with two-dimensional rectangular items. The loading area of the vehicles is also a rectangular area where the items are taken out from one side. A constraint states that the items of a given customer must be directly accessible at delivery time.
The last paper considers a stochastic vehicle routing and loading problem with two- dimensional rectangular items where the dimensions of some items are unknown when the routes are planned. However, it is possible to associate a discrete probability distribution on the dimensions of these items. The problem is solved with the Integer L-Shaped method
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