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17 results on '"Pham, Hoang Giang"'

1. Outer Approximation and Super-modular Cuts for Constrained Assortment Optimization under Mixed-Logit Model

Author

Pham, Hoang Giang and Mai, Tien

Subjects
Mathematics - Optimization and Control, Computer Science - Artificial Intelligence
Abstract

In this paper, we study the assortment optimization problem under the mixed-logit customer choice model. While assortment optimization has been a major topic in revenue management for decades, the mixed-logit model is considered one of the most general and flexible approaches for modeling and predicting customer purchasing behavior. Existing exact methods have primarily relied on mixed-integer linear programming (MILP) or second-order cone (CONIC) reformulations, which allow for exact problem solving using off-the-shelf solvers. However, these approaches often suffer from weak continuous relaxations and are slow when solving large instances. Our work addresses the problem by focusing on components of the objective function that can be proven to be monotonically super-modular and convex. This allows us to derive valid cuts to outer-approximate the nonlinear objective functions. We then demonstrate that these valid cuts can be incorporated into Cutting Plane or Branch-and-Cut methods to solve the problem exactly. Extensive experiments show that our approaches consistently outperform previous methods in terms of both solution quality and computation time.

Published
2024

2. Competitive Facility Location under Random Utilities and Routing Constraints

Author

Pham, Hoang Giang, Dam, Tien Thanh, Duong, Ngan Ha, Mai, Tien, and Ha, Minh Hoang

Subjects
Computer Science - Artificial Intelligence
Abstract

In this paper, we study a facility location problem within a competitive market context, where customer demand is predicted by a random utility choice model. Unlike prior research, which primarily focuses on simple constraints such as a cardinality constraint on the number of selected locations, we introduce routing constraints that necessitate the selection of locations in a manner that guarantees the existence of a tour visiting all chosen locations while adhering to a specified tour length upper bound. Such routing constraints find crucial applications in various real-world scenarios. The problem at hand features a non-linear objective function, resulting from the utilization of random utilities, together with complex routing constraints, making it computationally challenging. To tackle this problem, we explore three types of valid cuts, namely, outer-approximation and submodular cuts to handle the nonlinear objective function, as well as sub-tour elimination cuts to address the complex routing constraints. These lead to the development of two exact solution methods: a nested cutting plane and nested branch-and-cut algorithms, where these valid cuts are iteratively added to a master problem through two nested loops. We also prove that our nested cutting plane method always converges to optimality after a finite number of iterations. Furthermore, we develop a local search-based metaheuristic tailored for solving large-scale instances and show its pros and cons compared to exact methods. Extensive experiments are conducted on problem instances of varying sizes, demonstrating that our approach excels in terms of solution quality and computation time when compared to other baseline approaches.

Published
2024

9. A new constraint programming model and a linear programming-based adaptive large neighborhood search for the vehicle routing problem with synchronization constraints

Author

Hà, Minh Hoàng, Nguyen, Tat Dat, Duy, Thinh Nguyen, Pham, Hoang Giang, Do, Thuy, and Rousseau, Louis-Martin

Subjects
Computer Science - Artificial Intelligence
Abstract

We consider a vehicle routing problem which seeks to minimize cost subject to time window and synchronization constraints. In this problem, the fleet of vehicles is categorized into regular and special vehicles. Some customers require both vehicles' services, whose starting service times at the customer are synchronized. Despite its important real-world application, this problem has rarely been studied in the literature. To solve the problem, we propose a Constraint Programming (CP) model and an Adaptive Large Neighborhood Search (ALNS) in which the design of insertion operators is based on solving linear programming (LP) models to check the insertion feasibility. A number of acceleration techniques is also proposed to significantly reduce the computational time. The computational experiments show that our new CP model finds better solutions than an existing CP-based ANLS, when used on small instances with 25 customers and with a much shorter running time. Our LP-based ALNS dominates the cp-ALNS, in terms of solution quality, when it provides solutions with better objective values, on average, for all instance classes. This demonstrates the advantage of using linear programming instead of constraint programming when dealing with a variant of vehicle routing problems with relatively tight constraints, which is often considered to be more favorable for CP-based methods.

Published
2019

14. The Efficiency of Air-lift Bioreactor Using PVA-gel Carrier Materials to Treat Organic Components in Artificial Wastewater

Author

Tran Manh Hai, Le Van Chieu, Pham Hoang Giang, Vu Ngoc Duy, and Nguyen Truong Quan

Abstract

An air-lift bioreactor using PVA-gel was used to remove organic components in wastewater treatment. The wastewater treatment system using Sequencing Batch Reactor with carrier material volume of 5%, activated sludge density of 2.5 g/L to treat artificial wastewater made from cat food (Whiskas) with initial COD content of 2,000 mg/L. After 10 days of operation, the removal efficiency of total COD, soluble COD, and achieved 92, 89, and 84%, respectively. Despite the high removal efficiencies of organic components, the output wastewater did not achieve QCVN 40:2011/BTNMT (Column B) discharge standards. Due to high concentrations of TSS and slow biodegradable organic components. The experimental system will be necessary to study the operating conditions to treat wastewater more efficiently and achieve the discharge standards to the environment.

Published
2022

15. A rolling horizon approach for the locomotive routing problem at the Canadian National Railway Company

Author

Pham, Hoang Giang, Frejinger, Emma, and Cordeau, Jean-François

Subjects
Approche par Horizon Roulant, Locomotive Routing Problem, Rolling Horizon Approach, Problème du Routage des Locomotives
Abstract

Cette thèse étudie le problème du routage des locomotives qui se pose à la Compagnie des chemins de fer nationaux du Canada (CN) - le plus grand chemin de fer au Canada en termes de revenus et de taille physique de son réseau ferroviaire. Le problème vise à déterminer la séquence des activités de chaque locomotive sur un horizon de planification donné. Dans ce contexte, il faut prendre des décisions liées à l'affectation de locomotives aux trains planifiés en tenant compte des besoins d'entretien des locomotives. D’autres décisions traitant l'envoi de locomotives aux gares par mouvements à vide, les déplacements légers (sans tirer des wagons) et la location de locomotives tierces doivent également être prises en compte. Sur la base d'une formulation de programmation en nombres entiers et d'un réseau espace-temps présentés dans la littérature, nous introduisons une approche par horizon roulant pour trouver des solutions sous-optimales de ce problème dans un temps de calcul acceptable. Une formulation mathématique et un réseau espace-temps issus de la littérature sont adaptés à notre problème. Nous introduisons un nouveau type d'arcs pour le réseau et de nouvelles contraintes pour le modèle pour faire face aux problèmes qui se posent lors de la division de l'horizon de planification en plus petits morceaux. Les expériences numériques sur des instances réelles montrent les avantages et les inconvénients de notre algorithme par rapport à une approche exacte., This thesis addresses the locomotive routing problem arising at the Canadian National Railway Company (CN) - the largest railway in Canada in terms of both revenue and the physical size of its rail network. The problem aims to determine the sequence of activities for each locomotive over the planning horizon. Besides assigning locomotives to scheduled trains and considering scheduled locomotive maintenance requirements, the problem also includes other decisions, such as sending locomotives to stations by deadheading, light traveling, and leasing of third-party locomotives. Based on an Integer Programming formulation and a Time-Expanded Network presented in the literature, we introduce a Rolling Horizon Approach (RHA) as a method to find near-optimal solutions of this problem in acceptable computing time. We adapt a mathematical formulation and a space-time network from the literature. We introduce a new type of arcs for the network and new constraints for the model to cope with issues arising when dividing the planning horizon into smaller ones. Computational experiments on real-life instances show the pros and cons of our algorithm when compared to an exact solution approach.

Published
2020

17. A Study of the Adsorption Characteristics of Cobalt and Caesium from a Solution by Using Vietnamese Bentonite

Author

Pham Hoang Giang, Bui Dang Hanh, Le Phuoc Cuong, and Gergő Bátor

Subjects
Radionuclide, Waste management, General Chemical Engineering, chemistry.chemical_element, Radioactive waste, General Chemistry, law.invention, Adsorption, chemistry, Safe operation, Chemistry (miscellaneous), law, Caesium, Bentonite, Nuclear power plant, Cobalt
Abstract

The radioactive waste produced from the construction of a nuclear power plant is a controversial topic. The resulting radioactive waste contains 60Co and 137Cs isotopes that are the most difficult to remove. Bentonite is widely used as an adsorbent for heavy metals. An important factor is the safe operation of waste management at a nuclear power plant to be built in Vietnam. Therefore, a method of degrading complexes of radionuclides and the adsorption of radionuclides onto Vietnamese Bentonite was implemented in this study. In current literature, UV radiation and heating with oxidising substances are used in general for degrading complexes of radionuclides. The experimental results for the adsorption of Co(II) and Cs+ onto VNB suggest that VNB can be used in the future for large-scale liquid waste treatment due to its low cost, high efficiency, and environmentally friendliness.

Published
2015

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