Discrete Mathematics Group

Kyeongsik Nam (남경식), Random walks on percolation
2024-10-15 16:30 - 17:30 (Asia/Seoul) | Room B332, IBS (기초과학연구원)
In general, random walks on fractal graphs are expected to exhibit anomalous behaviors, for example heat kernel is significantly different from that in the case of lattices. Alexander and Orbach in 1982 conjectured that random walks on critical percolation, a prominent example of fractal graphs, exhibit mean field behavior; for instance, its spectral dimension is 4/3. In this talk, I will talk about this conjecture for a canonical dependent percolation model.

Colin Geniet, Permutations, patterns, and twin-width
2024-10-22 16:30 - 17:30
This talk will first introduce combinatorics on permutations and patterns, presenting the basic notions and some fundamental results: the Marcus-Tardos theorem which bounds the density of matrices avoiding a given pattern, and the Guillemot-Marx algorithm for pattern detection using the notion now known as twin-width.
I will then present a decomposition result: permutations avoiding a pattern factor into bounded products of separable permutations. This can be rephrased in terms of twin-width: permutation with bounded twin-width are build from a bounded product of permutations of twin-width 0. Comparable results on graph encodings follow from this factorisation.
This is joint work with Édouard Bonnet, Romain Bourneuf, and Stéphan Thomassé.

Felix Christian Clemen, Triangles in the Plane
2024-10-29 16:30 - 17:30
A classical problem in combinatorial geometry, posed by Erdős in 1946, asks to determine the maximum number of unit segments in a set of $n$ points in the plane. Since then a great variety of extremal problems in finite planar point sets have been studied. Here, we look at such questions concerning triangles. Among others we answer the following question asked by Erdős and Purdy almost 50 years ago: Given $n$ points in the plane, how many triangles can be approximate congruent to equilateral triangles?
For our proofs we use hypergraph Turán theory. This is joint work with Balogh and Dumitrescu.

Michał Pilipczuk, Monadic stability and monadic dependence
2024-11-05 16:30 - 17:30
We will give an overview of the recent attempts of building a structure theory for graphs centered around First-Order transductions: a notion of containment inspired by finite model theory. Particularly, we will speak about the notions of monadic dependence and monadic stability, their combinatorial characterizations, and the developments on the algorithmic front.

Karim Adiprasito, Ehrhart theory revisited: Algebraic aspects, unimodality and more
2024-11-12 16:30 - 17:30
Ehrhart theory is the study of lattice polytopes, specifically aimed at understanding how many lattice points are inside dilates of a given lattice polytope, and the study has a wide range of connections ranging from coloring graphs to mirror symmetry and representation theory. Recently, we introduced new algebraic tools to understand this theory, and resolve some classical conjectures. I will explain the combinatorial underpinnings behind two of the key techniques: Parseval identities for semigroup algebras, and the character algebra of a semigroup.

Eng Keat Hng, Graphon branching processes and fractional isomorphism
2024-11-26 16:30 - 17:30
In 2005, Bollobás, Janson and Riordan introduced and extensively studied a general model of inhomogeneous random graphs parametrised by graphons. In particular, they studied the emergence of a giant component in these inhomogeneous random graphs by relating them to a broad collection of inhomogeneous Galton-Watson branching processes.
Fractional isomorphism of finite graphs is an important and well-studied concept at the intersection of graph theory and combinatorial optimisation. It has many different characterizations that involve a range of very different and seemingly unrelated properties of graphs. Recently, Grebík and Rocha developed a theory of fractional isomorphism for graphons.
In our work, we characterise inhomogeneous random graphs that yield the same inhomogeneous Galton-Watson branching process (and hence have a similar component structure).
This is joint work with Jan Hladký and Anna Margarethe Limbach.

Yulai Ma, Pairwise disjoint perfect matchings in regular graphs
2024-12-03 16:30 - 17:30
An $r$-graph is an $r$-regular graph in which every odd set of vertices is connected to its complement by at least $r$ edges. A central question regarding $r$-graphs is determining the maximum number of pairwise disjoint perfect matchings they can contain. This talk explores how edge connectivity influences this parameter.
For ${0 leq lambda leq r}$, let $m(lambda,r)$ denote the maximum number $s$ such that every $lambda$-edge-connected $r$-graph contains $s$ pairwise disjoint perfect matchings. The values of $m(lambda,r)$ are known only in limited cases; for example, $m(3,3)=m(4,r)=1$, and $m(r,r) leq r-2$ for all $r
ot = 5$, with $m(r,r) leq r-3$ when $r$ is a multiple of $4$. In this talk, we present new upper bounds for $m(lambda,r)$ and examine connections between $m(5,5)$ and several well-known conjectures for cubic graphs.
This is joint work with Davide Mattiolo, Eckhard Steffen, and Isaak H. Wolf.

Jun Gao (高峻), Phase transition of degenerate Turán problems in p-norms
2024-12-13 16:30 - 17:30
For a positive real number $p$, the $p$-norm $|G|_p$ of a graph $G$ is the sum of the $p$-th powers of all vertex degrees. We study the maximum $p$-norm $mathrm{ex}_{p}(n,F)$ of $F$-free graphs on $n$ vertices, focusing on the case where $F$ is a bipartite graph. It is natural to conjecture that for every bipartite graph $F$, there exists a threshold $p_F$ such that for $p< p_{F}$, the order of $mathrm{ex}_{p}(n,F)$ is governed by pseudorandom constructions, while for $p > p_{F}$, it is governed by star-like constructions. We determine the exact value of $p_{F}$, under a mild assumption on the growth rate of $mathrm{ex}(n,F)$. Our results extend to $r$-uniform hypergraphs as well.
We also prove a general upper bound that is tight up to a $log n$ factor for $mathrm{ex}_{p}(n,F)$ when $p = p_{F}$.
We conjecture that this $log n$ factor is unnecessary and prove this conjecture for several classes of well-studied bipartite graphs, including one-side degree-bounded graphs and families of short even cycles.
This is a joint work with Xizhi Liu, Jie Ma and Oleg Pikhurko.

Joonkyung Lee (이준경), Counting homomorphisms in antiferromagnetic graphs via Lorentzian polynomials
2024-12-17 16:30 - 17:30
An edge-weighted graph $G$, possibly with loops, is said to be antiferromagnetic if it has nonnegative weights and at most one positive eigenvalue, counting multiplicities. The number of graph homomorphisms from a graph $H$ to an antiferromagnetic graph $G$ generalises various important parameters in graph theory, including the number of independent sets and proper vertex colourings.
We obtain a number of new homomorphism inequalities for antiferromagnetic target graphs $G$. In particular, we prove that, for any antiferromagnetic $G$,
$|mathrm{Hom}(K_d, G)|^{1/d} ≤ |mathrm{Hom}(K_{d,d} setminus M, G)|^{1/(2d)}$
holds, where $K_{d,d} setminus M$ denotes the complete bipartite graph $K_{d,d}$ minus a perfect matching $M$. This confirms a conjecture of Sah, Sawhney, Stoner and Zhao for complete graphs $K_d$. Our method uses the emerging theory of Lorentzian polynomials due to Brändén and Huh, which may be of independent interest.
Joint work with Jaeseong Oh and Jaehyeon Seo.

Zixiang Xu (徐子翔), Multilinear polynomial methods and stability results on set systems
2024-12-23 16:30 - 17:30
In 1966, Kleitman established that if ( |A triangle B| leq d ) for any ( A, B in mathcal{F} ), then ( |mathcal{F}| leq sum_{i=0}^{k} binom{n}{i} ) for ( d = 2k ), and ( |mathcal{F}| leq 2 sum_{i=0}^{k} binom{n-1}{i} ) for ( d = 2k+1 ). These upper bounds are attained by the radius-(k) Hamming ball ( mathcal{K}(n, k) := { F : F subseteq [n], |F| leq k } ) in the even case, and by the family ( mathcal{K}_y(n, k) := { F : F subseteq [n], |F setminus {y}| leq k } ) in the odd case. In 2017, Frankl provided a combinatorial proof of a stability result for Kleitman’s theorem, offering improved upper bounds for ( |mathcal{F}| ) when ( mathcal{F} ) is not the extremal structure.
In this talk, I will begin by demonstrating the application of multilinear polynomial methods in extremal set theory, highlighting some interesting techniques. I will then present an algebraic proof of the stability result for Kleitman’s theorem. Finally, I will discuss further applications and explore how to employ linear algebra methods more effectively and flexibly.
This talk is based on joint work with Jun Gao and Hong Liu.

Huy Tuan Pham, Random Cayley graphs and Additive combinatorics without groups
2025-01-03 16:30 - 17:30
A major goal of additive combinatorics is to understand the structures of subsets A of an abelian group G which has a small doubling K = |A+A|/|A|. Freiman’s celebrated theorem first provided a structural characterization of sets with small doubling over the integers, and subsequently Ruzsa in 1999 proved an analog for abelian groups with bounded exponent. Ruzsa further conjectured the correct quantitative dependence on the doubling K in his structural result, which has attracted several interesting developments over the next two decades. I will discuss a complete resolution of (a strengthening of) Ruzsa’s conjecture.
Surprisingly, our approach is crucially motivated by purely graph-theoretic insights, where we find that the group structure is superfluous and can be replaced by much more general combinatorial structures. Using this general approach, we also obtain combinatorial and nonabelian generalizations of classical results in additive combinatorics, and solve longstanding open problems around Cayley graphs and random Cayley graphs motivated by Ramsey theory, information theory and computer science.
Based on joint work with David Conlon, Jacob Fox and Liana Yepremyan.

Tony Huynh, The Peaceable Queens Problem
2025-01-14 16:30 - 17:30
The peaceable queens problem asks to determine the maximum number $a(n)$ such that there is a placement of $a(n)$ white queens and $a(n)$ black queens on an $n times n$ chessboard so that no queen can capture any queen of the opposite color.
We consider the peaceable queens problem and its variant on the toroidal board. For the regular board, we show that $a(n) leq 0.1716n^2$, for all sufficiently large $n$. This improves on the bound $a(n) leq 0.25n^2$ of van Bommel and MacEachern.
For the toroidal board, we provide new upper and lower bounds. Somewhat surprisingly, our bounds show that there is a sharp contrast in behaviour between the odd torus and the even torus. Our lower bounds are given by explicit constructions. For the upper bounds, we formulate the problem as a non-linear optimization problem with at most 100 variables, regardless of the size of the board. We solve our non-linear program exactly using modern optimization software.
We also provide a local search algorithm and a software implementation which converges very rapidly to solutions which appear optimal. Our algorithm is sufficiently robust that it works on both the regular and toroidal boards. For example, for the regular board, the algorithm quickly finds the so-called Ainley construction.
This is joint work with Katie Clinch, Matthew Drescher, and Abdallah Saffidine.

Laure Morelle, Bounded size modifications in time $2^{{sf poly}(k)}cdot n^2$
2025-01-21 16:30 - 17:30
A replacement action is a function $mathcal L$ that maps each graph to a collection of subgraphs of smaller size.
Given a graph class $mathcal H$, we consider a general family of graph modification problems, called “$mathcal L$-Replacement to $mathcal H$”, where the input is a graph $G$ and the question is whether it is possible to replace some induced subgraph $H_1$ of $G$ on at most $k$ vertices by a graph $H_2$ in ${mathcal L}(H_1)$ so that the resulting graph belongs to $mathcal H$.
“$mathcal L$-Replacement to $mathcal H$” can simulate many graph modification problems including vertex deletion, edge deletion/addition/edition/contraction, vertex identification, subgraph complementation, independent set deletion, (induced) matching deletion/contraction, etc.
We prove here that, for any minor-closed graph class $mathcal H$ and for any action $mathcal L$ that is hereditary, there is an algorithm that solves “$mathcal L$-Replacement to $mathcal H$” in time $2^{{sf poly}(k)}cdot |V(G)|^2$, where $sf poly$ is a polynomial whose degree depends on $mathcal H$.

Jang Soo Kim (김장수), Longest elements in a semigroup of functions and Slater indices
2025-02-04 16:30 - 17:30
The group ( S_n ) of permutations on ([n]={1,2,dots,n} ) is generated by simple transpositions ( s_i = (i,i+1) ). The length ( ell(pi) ) of a permutation ( pi ) is defined to be the minimum number of generators whose product is ( pi ). It is well-known that the longest element in ( S_n ) has length ( n(n-1)/2 ). Let ( F_n ) be the semigroup of functions ( f:[n]to[n] ), which are generated by the simple transpositions ( s_i ) and the function ( t:[n]to[n] ) given by ( t(1) =t(2) = 1 ) and ( t(i) = i ) for ( ige3 ). The length ( ell(f) ) of a function ( fin F_n ) is defined to be the minimum number of these generators whose product is ( f ). In this talk, we study the length of longest elements in ( F_n ). We also find a connection with the Slater index of a tournament of the
complete graph ( K_n ). This is joint work with Yasuhide Numata.

Jungho Ahn (안정호), A coarse Erdős-Pósa theorem for constrained cycles
2025-02-11 16:30 - 17:30
An induced packing of cycles in a graph is a set of vertex-disjoint cycles such that the graph has no edge between distinct cycles of the set. The classic Erdős-Pósa theorem shows that for every positive integer $k$, every graph contains $k$ vertex-disjoint cycles or a set of $O(klog k)$ vertices which intersects every cycle of $G$.
We generalise this classic Erdős-Pósa theorem to induced packings of cycles of length at least $ell$ for any integer $ell$. We show that there exists a function $f(k,ell)=O(ell klog k)$ such that for all positive integers $k$ and $ell$ with $ellgeq3$, every graph $G$ contains an induced packing of $k$ cycles of length at least $ell$ or a set $X$ of at most $f(k,ell)$ vertices such that the closed neighbourhood of $X$ intersects every cycle of $G$.
Furthermore, we extend the result to long cycles containing prescribed vertices. For a graph $G$ and a set $Ssubseteq V(G)$, an $S$-cycle in $G$ is a cycle containing a vertex in $S$. We show that for all positive integers $k$ and $ell$ with $ellgeq3$, every graph $G$, and every set $Ssubseteq V(G)$, $G$ contains an induced packing of $k$ $S$-cycles of length at least $ell$ or a set $X$ of at most $ell k^{O(1)}$ vertices such that the closed neighbourhood of $X$ intersects every cycle of $G$.
Our proofs are constructive and yield polynomial-time algorithms, for fixed $ell$, finding either the induced packing of the constrained cycles or the set $X$.
This is based on joint works with Pascal Gollin, Tony Huynh, and O-joung Kwon.

O-joung Kwon (권오정), Erdős-Pósa property of A-paths in unoriented group-labelled graphs
2025-02-18 16:30 - 17:30
A family $mathcal{F}$ of graphs is said to satisfy the Erdős-Pósa property if there exists a function $f$ such that for every positive integer $k$, every graph $G$ contains either $k$ (vertex-)disjoint subgraphs in $mathcal{F}$ or a set of at most $f(k)$ vertices intersecting every subgraph of $G$ in $mathcal{F}$. We characterize the obstructions to the Erdős-Pósa property of $A$-paths in unoriented group-labelled graphs. As a result, we prove that for every finite abelian group $Gamma$ and for every subset $Lambda$ of $Gamma$, the family of $Gamma$-labelled $A$-paths whose lengths are in $Lambda$ satisfies the half-integral relaxation of the Erdős-Pósa property. Moreover, we give a characterization of such $Gamma$ and $LambdasubseteqGamma$ for which the same family of $A$-paths satisfies the full Erdős-Pósa property. This is joint work with Youngho Yoo.

Sepehr Hajebi, The pathwidth theorem for induced subgraphs
2025-02-25 16:30 - 17:30
We present a full characterization of the unavoidable induced subgraphs of graphs with large pathwidth. This consists of two results. The first result says that for every forest H, every graph of sufficiently large pathwidth contains either a large complete subgraph, a large complete bipartite induced minor, or an induced minor isomorphic to H. The second result describes the unavoidable induced subgraphs of graphs with a large complete bipartite induced minor.
We will also try to discuss the proof of the first result with as much detail as time permits.
Based on joint work with Maria Chudnovsky and Sophie Spirkl.

Irene Muzi, An elementary bound for Younger's conjecture
2025-03-04 16:30 - 17:30
In 1996, Reed, Robertson, Seymour and Thomas proved Younger’s Conjecture, which states that for all directed graphs D, there exists a function f such that if D does not contain k disjoint cycles, D contains a feedback vertex set, i.e. a subset of vertices whose deletion renders the graph acyclic, of size bounded by f(k). However, the function obtained by Reed, Robertson, Seymour and Thomas is enormous and, in fact, not even elementary. The bound had, so far, not been improved. In this talk, I will present new techniques to improve the bound from non-elementary to elementary. This is joint work with Meike Hatzel, Stephan Kreutzer and Marcelo Milani.

Johannes Carmesin, Open problems in graph theory
2025-03-11 16:30 - 17:30
Since the proof of the graph minor structure theorem by Robertson and Seymour in 2004, its underlying ideas have found applications in a much broader range of settings than their original context. They have driven profound progress in areas such as vertex minors, pivot minors, matroids, directed graphs, and 2-dimensional simplicial complexes. In this talk, I will present three open problems related to this development, each requiring some background.

Michał Seweryn, Dimension and standard examples in planar posets
2025-03-18 16:30 - 17:30
The dimension of a poset is the least integer $d$ such that the poset is isomorphic to a subposet of the product of $d$ linear orders. In 1983, Kelly constructed planar posets of arbitrarily large dimension. Crucially, the posets in his construction involve large standard examples, the canonical structure preventing a poset from having small dimension. Kelly’s construction inspired one of the most challenging questions in dimension theory: are large standard examples unavoidable in planar posets of large dimension? We answer the question affirmatively by proving that every $d$-dimensional planar poset contains a standard example of order $Omega(d)$. More generally, we prove that every poset from Kelly’s construction appears in every poset with a planar cover graph of sufficiently large dimension.
joint work with Heather Smith Blake, Jędrzej Hodor, Piotr Micek, and William T. Trotter.

Hyunwoo Lee (이현우), Reconstructing hypergraph matching polynomials
2025-04-01 16:30 - 17:30
By utilizing the recently developed hypergraph analogue of Godsil’s identity by the second author, we prove that for all $n geq k geq 2$, one can reconstruct the matching polynomial of an $n$-vertex $k$-uniform hypergraph from the multiset of all induced sub-hypergraphs on $lfloor frac{k-1}{k}n rfloor + 1$ vertices. This generalizes the well-known result of Godsil on graphs in 1981 to every uniform hypergraph. As a corollary, we show that for every graph $F$, one can reconstruct the number of $F$-factors in a graph under analogous conditions. We also constructed examples that imply the number $lfloor frac{k-1}{k}n rfloor + 1$ is the best possible for all $ngeq k geq 2$ with $n$ divisible by $k$. This is joint work Donggyu Kim.

Marcelo Garlet Milani, Cycles of Well-Linked Sets and an Elementary Bound for the Directed Grid Theorem
2025-04-08 16:30 - 17:30
In 2015, Kawarabayashi and Kreutzer proved the directed grid theorem. The theorem states the existence of a function f such that every digraph of directed tree-width f(k) contains a cylindrical grid of order k as a butterfly minor, but the given function grows non-elementarily with the size of the grid minor.
We present an alternative proof of the directed grid theorem, first proven by Kawarabayashi and Kreutzer in 2015, which is conceptually much simpler, more modular in its composition and also improves the upper bound for the function f to a power tower of height 22.
Our proof is inspired by the breakthrough result of Chekuri and Chuzhoy, who proved a polynomial bound for the excluded grid theorem for undirected graphs. We translate a key concept of their proof to directed graphs by introducing cycles of well-linked sets (CWS), and show that any digraph of high directed tree-width contains a large CWS, which in turn contains a large cylindrical grid.
This is joint work with Meike Hatzel, Stephan Kreutzer and Irene Muzi.

Daniel McGinnis, A necessary and sufficient condition for $k$-transversals
2025-04-14 11:00 - 12:00
We solve a long-standing open problem posed by Goodman and Pollack in 1988 by establishing a necessary and sufficient condition for a finite family of convex sets in $mathbb{R}^d$ to admit a $k$-transversal (a $k$-dimensional affine subspace that intersects each set) for any $0 le k le d-1$. This result is a common generalization of Helly’s theorem ($k=0$) and the Goodman-Pollack-Wenger theorem ($k=d-1$).
This is joint work with Nikola Sadovek.

Nicola Lorenz, A Minor Characterisation of Normally Spanned Sets of Vertices
2025-04-15 16:30 - 17:30
A rooted spanning tree of a graph $G$ is called normal if the endvertices of all edges of $G$ are comparable in the tree order. It is well known that every finite connected graph has a normal spanning tree (also known as depth-first search tree). Also, all countable graphs have normal spanning trees, but uncountable complete graphs for example do not. In 2021, Pitz proved the following characterisation for graphs with normal spanning trees, which had been conjectured by Halin: A connected graph $G$ has a normal spanning tree if and only if every minor of $G$ has countable colouring number, i.e. there is a well-order of the vertices such that every vertex is preceded by only finitely many of its neighbours.
More generally, a not necessarily spanning tree in $G$ is called normal if for every path $P$ in $G$ with both endvertices in $T$ but no inner vertices in $T$, the endvertices of $P$ are comparable in the tree order. We establish a local version of Pitz’s theorem by characterising for which sets $U$ of vertices of $G$ there is a normal tree in $G$ covering $U$. The results are joint work with Max Pitz.

Marcin Briański, Burling Graphs as (almost) universal obstacles to $chi$-boundedness
2025-04-22 16:30 - 17:30
What causes a graph to have high chromatic number? One obvious reason is containing a large clique (a set of pairwise adjacent vertices). This naturally leads to investigation of (chi)-bounded classes of graphs — classes where a large clique is essentially the only reason for large chromatic number.
Unfortunately, many interesting graph classes are not (chi)-bounded. An eerily common obstruction to being (chi)-bounded are the Burling graphs — a family of triangle-free graphs with unbounded chromatic number. These graphs have served as counterexamples in many settings: demonstrating that graphs excluding an induced subdivision of (K_{5}) are not (chi)-bounded, that string graphs are not (chi)-bounded, that intersection graphs of boxes in ({mathbb{R}}^{3}) are not (chi)-bounded, and many others.
In many of these cases, this sequence is the only known obstruction to (chi)-boundedness. This led Chudnovsky, Scott, and Seymour to conjecture that any graph of sufficiently high chromatic number must either contain a large clique, an induced proper subdivision of a clique, or a large Burling graph as an induced subgraph.
The prevailing belief was that this conjecture should be false. Somewhat surprisingly, we did manage to prove it under an extra assumption on the “locality” of the chromatic number — that the input graph belongs to a (2)-controlled family of graphs, where a high chromatic number is always certified by a ball of radius (2) with large chromatic number.
In this talk, I will present this result and discuss its implications in structural graph theory, and algorithmic implications to colouring problems in specific graph families.
This talk is based on joint work with Tara Abrishami, James Davies, Xiying Du, Jana Masaříková, Paweł Rzążewski, and Bartosz Walczak conducted during the STWOR2 workshop in Chęciny Poland.

Pascal Schweitzer, Recent insights surrounding combinatorial approaches to isomorphism and symmetry problems
2025-04-28 11:00 - 12:00
Modern practical software libraries that are designed for isomorphism tests and symmetry computation rely on combinatorial techniques combined with techniques from algorithmic group theory. The Weisfeiler-Leman algorithm is such a combinatorial technique. When taking a certain view from descriptive complexity theory, the algorithm is universal. After an introduction to problems arising in symmetry computation and this particular combinatorial technique, I will give an overview of results from recent years. The results give insights into worst-case behavior and computational complexity.

Eunjin Oh (오은진), Approximation Algorithms for the Geometric Multimatching Problem
2025-04-29 16:30 - 17:30
Let S and T be two sets of points in a metric space with a total of n points. Each point in S and T has an associated value that specifies an upper limit on how many points it can be matched with from the other set. A multimatching between S and T is a way of pairing points such that each point in S is matched with at least as many points in T as its assigned value, and vice versa for each point in T. The cost of a multimatching is defined as the sum of the distances between all matched pairs of points. The geometric multimatching problem seeks to find a multimatching that minimizes this cost. A special case where each point is matched to at most one other point is known as the geometric many-to-many matching problem.
We present two results for these problems when the underlying metric space has a bounded doubling dimension. Specifically, we provide the first near-linear-time approximation scheme for the geometric multimatching problem in terms of the output size. Additionally, we improve upon the best-known approximation algorithm for the geometric many-to-many matching problem, previously introduced by Bandyapadhyay and Xue (SoCG 2024), which won the best paper award at SoCG 2024.
This is joint work with Shinwoo An and Jie Xue.

Seokbeom Kim (김석범), The structure of △(1, 2, 2)-free tournaments
2025-05-13 16:30 - 17:30
Given a tournament $S$, a tournament is $S$-free if it has no subtournament isomorphic to $S$. Until now, there have been only a small number of tournaments $S$ such that the complete structure of $S$-free tournaments is known.
Let $triangle(1, 2, 2)$ be a tournament obtained from the cyclic triangle by substituting two-vertex tournaments for two of its vertices. In this talk, we present a structure theorem for $triangle(1, 2, 2)$-free tournaments, which was previously unknown. As an application, we provide tight bounds for the chromatic number as well as the size of the largest transitive subtournament for such tournaments.
This talk is based on joint work with Taite LaGrange, Mathieu Rundström, Arpan Sadhukhan, and Sophie Spirkl.

Meike Hatzel, Counterexample to Babai's lonely colour conjecture
2025-05-27 16:30 - 17:30
Motivated by colouring minimal Cayley graphs, in 1978 Babai conjectured that no-lonely-colour graphs have bounded chromatic number. We disprove this in a strong sense by constructing graphs of arbitrarily large girth and chromatic number that have a proper edge colouring in which each cycle contains no colour exactly once.
The result presented is the joint work with James Davies and Liana Yepremyan.

Denys Bulavka, Strict Erdős-Ko-Rado Theorems for Simplicial Complexes
2025-06-04 16:30 - 17:30
The now classical theorem of Erdős, Ko and Rado establishes
the size of a maximal uniform family of pairwise-intersecting sets as well as a characterization of the families attaining such upper bound. One natural extension of this theorem is that of restricting the possiblechoices for the sets. That is, given a simplicial complex, what is the size of a maximal uniform family of pairwise-intersecting faces. Holroyd and Talbot, and Borg conjectured that the same phenomena as in the classical case (i.e., the simplex) occurs: there is a maximal size pairwise-intersecting family with all its faces having some common vertex. Under stronger hypothesis, they also conjectured that if a family attains such bound then its members must have a common vertex. In this talk I will present some progress towards the characterization of the maximal families. Concretely I will show that the conjecture is true for near-cones of sufficiently high depth. In particular, this implies that the characterization of maximal families holds for, for example, the independence complex of a chordal graph with an isolated vertex as well as the independence complex of a (large enough) disjoint union of graphs with at least one isolated vertex. Under stronger hypothesis, i.e., more isolated vertices, we also recover a stability theorem.
This talk is based on a joint work with Russ Woodroofe.
