Portrait of Nagisa Sugishita

Nagisa Sugishita

Postdoctorate - HEC Montréal
Supervisor
Margarida Carvalho
Research Topics
Fairness
Game Theory
Multi-Agent Systems
Optimization

Publications

Unboundedness in Bilevel Optimization
Bárbara Rodrigues
Margarida Carvalho
Miguel Anjos
Nagisa Sugishita
Abstract Bilevel optimization has garnered growing interest over the past decade. However, little attention has been paid to detecting and d… (see more)ealing with unboundedness in these problems, with most research assuming a bounded high-point relaxation. In this paper, we address unboundedness in bilevel and multilevel optimization by studying its computational complexity. We show that deciding whether an optimistic linear bilevel problem is unbounded is strongly NP-complete, even without coupling constraints. Furthermore, we extend the hardness result to the linear multilevel case, by showing that for each extra level added, the decision problem of checking unboundedness moves up a level in the polynomial hierarchy. Deciding unboundedness of a mixed-integer multilevel problem is shown to be one level higher in the polynomial complexity hierarchy than the decision problem for linear multilevel problem with the same number of levels. Finally, we introduce two algorithmic approaches to determine whether a linear bilevel problem is unbounded and, if so, return a certificate of unboundedness. This certificate consists of a direction of unboundedness and corresponding bilevel feasible point. We present a proof of concept of these algorithmic approaches on some relevant examples, and provide a brief computational comparison.
2026-03-25
Mathematical Programming (published)
doi.org
The Strength of Fuel Refueling Location Problem Formulations
Nagisa Sugishita
Margarida Carvalho
Ribal Atallah
2024-09-05
ArXiv (preprint)
arxiv.org
arxiv.org