Lucas Létocart

Minimal multicut and maximal integer multiflow: A survey

We present a survey about the maximum integral multiflow and minimum multicut problems and their subproblems, such as the multiterminal cut and the unsplittable flow problems. We consider neither continuous multiflow nor minimum cost multiflow. Most of the results are very recent and some are new. We recall the dual relationship between both problems, give complexity results and algorithms, firstly in unrestricted graphs and secondly in several special graphs: trees, bipartite or planar graphs. A table summarizes the most important results.

A matheuristic approach for solving a home health care problem

Abstract We deal with a Home Health Care Problem (HHCP) which objective consists in constructing the optimal routes and rosters for the health care staffs. The challenge lies in combining aspects of vehicle routing and staff rostering which are two well known hard combinatorial optimization problems. To solve this problem, we initially propose an integer linear programming formulation (ILP) and we tested this model on small instances. To deal with larger instances we develop a matheuristic based on the decomposition of the ILP formulation into two problems. The first one is a set partitioning like problem and it represents the rostering part. The second problem consists in the routing part. This latter is equivalent to a Multi-depot Traveling Salesman Problem with Time Windows (MTSPTW).

The Multicommodity-Ring Location Routing Problem

The multicommodity-ring location routing problem (MRLRP) studied in this paper is an NP-hard minimization problem arising in city logistics. The aim is to locate a set of urban distribution centers (UDCs) and to connect them via a ring in which massive flows of goods will circulate. Goods are transported from gates located outside the city to a UDC, and either join a second UDC through the ring before being delivered in electric vans to the final customers or are delivered directly to the customers from the first UDC. The reverse trip with pickup and transportation to the gates is also possible. A delivery service path starts at a particular UDC, then visits a subset of customers and ends at the same UDC, another UDC, or a self-service parking lot (SPL). A pickup route can start from an SPL or a UDC and ends at a UDC. The objective is to minimize the sum of the installation costs of the ring, flow transportation costs, and routing costs. The MRLRP belongs to the class of location-routing problems (LRP). We model it with a set-partitioning-like representation of delivery and pickup trips and arc-flow elements to describe goods transportation in the ring and between the ring and the gates. We present three approaches to solving the MRLRP: an exact method for small-size instances, a matheuristic for instances of a larger size, and a hybrid approach that applies the exact method to the columns output by the matheuristic. Numerical results are provided for an exhaustive set of instances, obtained by extending benchmark instances of the capacitated LRP with additional MRLRP features.

Reducing graphs in graph cut segmentation

In few years, graph cuts have become a leading method for solving a wide range of problems in computer vision. However, graph cuts involve the construction of huge graphs which sometimes do not fit in memory. Currently, most of the max-flow algorithms are impracticable to solve such large scale problems. In the image segmentation context, some authors have proposed heuristics [1, 2, 3, 4] to get round this problem. In this paper, we introduce a new strategy for reducing exactly graphs. During the creation of the graph, before creating a new node, we test if the node is really useful to the max-flow computation. The nodes of the reduced graph are typically located in a narrow band surrounding the object edges. Empirically, solutions obtained on the reduced graphs are identical to the solutions on the complete graphs. A parameter of the algorithm can be tuned to obtain smaller graphs when an exact solution is not needed. The test is quickly computed and the time required by the test is often compensated by the time that would be needed to create the removed nodes and the additional time required by the computation of the cut on the larger graph. As a consequence, we sometimes even save time on small scale problems.

Reoptimization in Lagrangian methods for the 0-1 quadratic knapsack problem

The 0-1 quadratic knapsack problem consists of maximizing a quadratic objective function subject to a linear capacity constraint. To exactly solve large instances of this problem with a tree search algorithm (e.g., a branch and bound method), the knowledge of good lower and upper bounds is crucial for pruning the tree but also for fixing as many variables as possible in a preprocessing phase. The upper bounds used in the best known exact approaches are based on Lagrangian relaxation and decomposition. It appears that the computation of these Lagrangian dual bounds involves the resolution of numerous 0-1 linear knapsack subproblems. Thus, taking this huge number of resolutions into account, we propose to embed reoptimization techniques for improving the efficiency of the preprocessing phase of the 0-1 quadratic knapsack resolution. Namely, reoptimization is introduced to accelerate each independent sequence of 0-1 linear knapsack problems induced by the Lagrangian relaxation as well as the Lagrangian decomposition. Numerous numerical experiments validate the relevance of our approach.

Solving the electricity production planning problem by a column generation based heuristic

This paper presents a heuristic method based on column generation for the EDF (Electricité De France) long-term electricity production planning problem proposed as subject of the ROADEF/EURO 2010 Challenge. This is to our knowledge the first-ranked method among those methods based on mathematical programming, and was ranked fourth overall. The problem consists in determining a production plan over the whole time horizon for each thermal power plant of the French electricity company, and for nuclear plants, a schedule of plant outages which are necessary for refueling and maintenance operations. The average cost of the overall outage and production planning, computed over a set of demand scenarios, is to be minimized. The method proceeds in two stages. In the first stage, dates for outages are fixed once for all for each nuclear plant. Data are aggregated with a single average scenario and reduced time steps, and a set-partitioning reformulation of this aggregated problem is solved for fixing outage dates with a heuristic based on column generation. The pricing problem associated with each nuclear plant is a shortest path problem in an appropriately constructed graph. In the second stage, the reload level is determined at each date of an outage, considering now all scenarios. Finally, the production quantities between two outages are optimized for each plant and each scenario by solving independent linear programming problems.

An improving dynamic programming algorithm to solve the shortest path problem with time windows

Abstract An efficient use of dynamic programming requires a substantial reduction of the number of labels. We propose in this paper an efficient way of reducing the number of labels saved and dominance computing time. Our approach is validated by experiments on shortest path problem with time windows instances.

Multicuts and integral multiflows in rings

We show how to solve in polynomial time the multicut and the maximum integral multiflow problems in rings. Moreover, we give linear-time procedures to solve both problems in rings with uniform capacities.

An efficient hybrid heuristic method for the 0-1 exact k-item quadratic knapsack problem

The 0-1 exact k-item quadratic knapsack problem (E - kQKP) consists of maximizing a quadratic function subject to two linear constraints: the first one is the classical linear capacity constraint; the second one is an equality cardinality constraint on the number of items in the knapsack. Most instances of this NP-hard problem with more than forty variables cannot be solved within one hour by a commercial software such as CPLEX 12.1. We propose therefore a fast and efficient heuristic method which produces both good lower and upper bounds on the value of the problem in reasonable time. Specifically, it integrates a primal heuristic and a semidefinite programming reduction phase within a surrogate dual heuristic. A large computational experiments over randomly generated instances with up to 200 variables validates the relevance of the bounds produced by our hybrid dual heuristic, which yields known optima (and prove optimality) in 90% (resp. 76%) within 100 seconds on the average.

A knowledge-driven bi-clustering method for mining noisy datasets

Bicluster discovery is an important task in various experimental domains. We propose here a new biclustering system COBIC, which combines graph algorithms with data mining methods to efficiently extract highly relevant and potentially overlapping biclusters. COBIC is based on maximum flow / minimum cut algorithms and is able to take into account background knowledge expressed as a classification, by a weight adaptation mechanism when iteratively extracting dense regions. The proposed approach, when compared on three real datasets (Yeast gene expression datasets) with recent and efficient biclustering algorithms shows very good performances.