Maria Chudnovsky, TBA
2026-06-02 16:30 - 17:30 (Asia/Seoul) | Room B332, IBS (기초과학연구원)

Martin Milanič, TBA
2026-06-09 16:30 - 17:30 | Room B332, IBS (기초과학연구원)

J. Pascal Gollin, TBA
2026-06-16 16:30 - 17:30 | Room B332, IBS (기초과학연구원)

Stefan Weltge, Multiplicative assignment with upgrades
2026-06-19 16:30 - 17:30 | Room B332, IBS (기초과학연구원)
We study a problem related to submodular function optimization and the exact matching problem for which we show a rather peculiar status: its natural LP-relaxation can have fractional optimal vertices, but there is always also an optimal integral vertex, which we can also compute in polynomial time. More specifically, we consider the multiplicative assignment problem with upgrades in which we are given a set of customers and suppliers and we seek to assign each customer to a different supplier. Each customer has a demand and each supplier has a regular and an upgraded cost for each unit demand provided to the respective assigned client. Our goal is to upgrade at most k suppliers and to compute an assignment in order to minimize the total resulting cost. This can be cast as the problem to compute an optimal matching in a bipartite graph with the additional constraint that we must select k edges from a certain group of edges, similar to selecting k red edges in the exact matching problem. Also, selecting the suppliers to be upgraded corresponds to maximizing a submodular set function under a cardinality constraint. Our result yields an efficient LP-based algorithm to solve our problem optimally. In addition, we provide also a purely strongly polynomial-time algorithm for it. As an application, we obtain exact algorithms for the upgrading variant of the problem to schedule jobs on identical or uniformly related machines in order to minimize their sum of completion times, i.e., where we may upgrade up to k jobs to reduce their respective processing times.
This is joint work with Alexander Armbruster, Lars Rohwedder, Andreas Wiese, and Ruilong Zhang.

2026 Workshop on Topological Combinatorics
2026-06-28 - 2026-07-11 | GIST
The 2026 Workshop on Topological Combinatorics will be held from June 28 to July 11, 2026 at Gwangju Institute of Science and Technology (GIST), located in Gwangju in the southwest of Republic of Korea.
The workshop aims to bring together researchers interested in applications of topology to combinatorics and related areas. This will be the fifth workshop in a series initiated by Ron Aharoni (August 2018 in Shantou, July 2019 in Prague, July 2020 on Zoom, and June 2024 in Paris).
The first week (June 28 – July 04) will be reserved for presentations, while the following days (July 05 – July 11) will be for discussion and collaborations.
The campus offers a variety of amenities, including a guesthouse that can accommodate approximately 20 guests.
In addition, various lodging options are available within a 15–20 minute walking distance.
We are planning to provide accommodation for all invited participants, and we are also considering offering partial travel support for those with limited funding.

Website: https://sites.google.com/view/2026topocomb

Ting-Wei Chao, The Oddtown Problem Modulo a Composite Number
2026-07-10 16:30 - 17:30 | Room B332, IBS (기초과학연구원)
A family of sets in $[n]$ is called an $ell$-Oddtown if the sizes of all sets are not divisible by $ell$, but the sizes of pairwise intersections are divisible by $ell$. The problem was completely solved when $ell$ is a prime via an elegant linear algebraic method, showing that the family has size at most $n$. However, not much was known for composite numbers. By splitting the family into families correspond to each prime factor of $ell$, one can show that the number is at most $omega n$, where $omega$ is the number of prime factors of $ell$. We used both combinatorial and Fourier analytic arguments to prove that the number of sets in any $ell$-Oddtown is at most $omega n-(2omega+varepsilon)log_2 n$ for most $n,ell$.

2026 Korean Student Combinatorics Workshop
2026-07-27 - 2026-07-31 | Gongju Hanok Village, Gongju
Website: https://kscw.combinatorics.kr/

Jinyoung Park (박진영), TBA
2026-08-18 16:30 - 17:30 | Room B332, IBS (기초과학연구원)
