Tom Demeulemeester

Peer-reviewed publications

Fair integer programming under dichotomous and cardinal preferences. (with Dries Goossens, Ben Hermans & Roel Leus).
European Journal of Operational Research, 320(3), 465-478. (2025). [pdf][link][code].

A pessimist’s approach to one-sided matching. (with Dries Goossens, Ben Hermans & Roel Leus).
European Journal of Operational Research, 305(3), 1087-1099. (2023). [pdf] [link] [slides] [data]

Strategyproofness and proportionality in party-approval multi-winner elections (with Théo Delemazure, Manuel Eberl, Jonas Israel & Patrick Lederer).
Proceedings of the 37th AAAI Conference on Artificial Intelligence (Vol. 37(5), 5591-5599). (2023). [pdf] [link]

Relaxed core stability in hedonic games with size-dependent utilities (with Jannik Peters).
48th International Symposium on Mathematical Foundations of Computer Science (MFCS 2023) (Vol. 272, 41:1-41:14). (2023). [pdf][code][link]

Working papers

Rawlsian assignments (with Juan Sebastián Pereyra) [pdf][code][arXiv]. Under submission.

We study the assignment of indivisible objects to individuals when transfers are not allowed. Previous literature has mainly focused on efficiency (from an ex-ante and ex-post perspective), and individually fair assignments. Consequently, egalitarian concerns have been overlooked. We are inspired by the assignment of apartments in housing cooperatives where families regard the egalitarianism of the assignments as a first-order requirement. In particular, they want to avoid assignments where some families get their most preferred apartment, while others get options ranked very low in their preferences. Based on Rawls' idea of fairness, we introduce the notion of Rawlsian assignments. We prove that there always exists a unique Rawlsian assignment, which is ordinally efficient, and satisfies equal treatment of equals. We illustrate our analysis with preference data from housing cooperatives. Our results show that the Rawlsian assignment substantially improves, from an egalitarian perspective, both the probabilistic serial mechanism, and the mechanism currently in use.

Quantifying core stability relaxations in hedonic games (with Jannik Peters) [pdf][arXiv][slides]. Under submission.

We study relationships between different relaxed notions of core stability in hedonic games, which are a class of coalition formation games. Our unified approach applies to a newly introduced family of hedonic games, called alpha-hedonic games, which contains previously studied variants such as fractional and additively separable hedonic games. In particular, we derive an upper bound on the maximum factor with which a blocking coalition of a certain size can improve upon an outcome in which no deviating coalition of size at most q exists. Counterintuitively, we show that larger blocking coalitions might sometimes have lower improvement factors. We discuss the tightness conditions of our bound, as well as its implications on the price of anarchy of core relaxations. Our general result has direct implications for several well-studied classes of hedonic games, allowing us to prove two open conjectures by Fanelli et al. (2021) for fractional hedonic games.

Other

Fairness through randomization: An operations research perspective. Doctoral dissertation. (2024). [pdf][slides]

Incorporating a certain level of randomization is a prerequisite for designing fair processes in many applications. In this dissertation, we illustrate that intuitive methods to incorporate randomization, such as randomly ordering the agents, can often be improved upon from various perspectives. We study three distinct, yet related problem settings, and apply techniques from the field of operations research to find and compute alternative solutions with desirable properties. First, we propose a new egalitarian solution concept for the random assignment problem, which studies how to assign indivisible goods to agents who have preferences over these goods, without making use of monetary transfers. Second, for an extension of the random assignment problem, we study how to decompose probabilistic solutions as a lottery over deterministic outcomes while optimizing a worst-case measure of choice. Third, we propose methods to fairly control the selection probabilities of the (possibly many) optimal solutions of an integer linear program. For each of the three evaluated problem settings, we evaluate both the axiomatic and the computational properties of our proposed solutions. Our proposed methods can be applied to a wide range of practical problems, such as the assignment of apartments in housing cooperatives, the assignment of students to schools or universities, or kidney exchange programs.

Reducing the negative effects of random tie-breaking in student allocation mechanisms. Master's thesis. (2019). [pdf]
Winner of the Ekonomika Master Thesis Award & Finalist of the ORBEL Award for best Master Thesis in Operations Research.

This thesis introduces two new methods to reduce the negative effects that are caused by using randomness as a selection criterion in a centralized allocation problem of students to schools. The first method, the Waste-Reducing Lottery Design (WRLD) procedure, reduces the number of available seats that are not assigned to any student due to random tie-breaking. The second method, the Maximin decomposition, reduces the uncertainty about the total number of students that will be assigned to a school. Both methods obtain their objective by determining the probability with which an allocation of students to schools will be selected as the final allocation. As both methods are applicable to all mechanisms that adopt random tie-breaking, they provide a general framework for decision-makers to improve upon the currently used mechanisms. The performance of the introduced methods is evaluated both on real-world data from Antwerp and Ghent, and on generated data.