2024 Combinatorics Workshop

Session

Contributed Talk

28 Aug 2024, 16:00

107 (Bldg. S1-6 (자연대6호관))

8. 102-avoiding Inversion Sequences

Heesung Shin (Inha University)

28/08/2024, 16:00

A sequence $(e_1,e_2,\cdots ,e_n)$ is an inversion sequences if $0\le e_i<i$ for all $i=1,\dots ,n$. We say that an inversion sequences $e=(e_1,e_2,\cdots ,e_n)$ \emph{contains} the pattern $102$ if there exist some indices $i<j<k$ such that $e_j<e_i<e_k$. Otherwise, $e$ is said to \emph{avoid} the pattern $102$.

In this talk, we will construct a correspondence between the...

7. Random matchings in linear hypergraphs

Hyunwoo Lee (KAIST & IBS ECOPRO)

28/08/2024, 16:30

For a given hypergraph $H$ and a vertex $v\in V(H)$, consider a random matching $M$ chosen uniformly from the set of all matchings in $H$. In 1995, Kahn conjectured that if $H$ is a $d$-regular linear $k$-uniform hypergraph, the probability that $M$ does not cover $v$ is $(1+o_d(1))d^{-1/k}$ for all vertices $v\in V(H)$. This conjecture was proved for $k=2$ by Kahn and Kim in 1998.
We...

3. Towards a classification of $1$-homogeneous graphs with positive intersection number $a_1$

Jae-Ho Lee (University of North Florida & POSTECH)

28/08/2024, 17:00

Let $\mathrm{\Gamma }$ be a graph with diameter at least two. Then $\mathrm{\Gamma }$ is said to be $1$-homogeneous (in the sense of Nomura) whenever for every pair of adjacent vertices $x$ and $y$ in $\mathrm{\Gamma }$, the distance partition of the vertex set of $\mathrm{\Gamma }$ with respect to both $x$ and $y$ is equitable, and the parameters corresponding to equitable partitions are independent of the choice of $x$ and $y$....

10. Transversal Hamilton paths and cycles of arbitrary orientations in tournaments

Jaehyeon Seo (Yonsei University)

29/08/2024, 11:00

It is well-known that a tournament always contains a directed Hamilton path. Rosenfeld conjectured that if a tournament is sufficiently large, it contains a Hamilton path of any given orientation. This conjecture was approved by Thomason, and Havet and Thomassé completely resolved it by showing there are exactly three exceptions.

We generalized this result into a transversal setting. Let...

2. Alternating $\mathcal{B}$-permutations arising from toric topology

Younghan Yoon (Ajou University)

29/08/2024, 15:00

In this talk, we focus on the rational Betti numbers of real toric manifolds associated with chordal nestohedra. We introduce an explicit description for the Betti numbers using alternating $\mathcal{B}$-permutations for a chordal building set $\mathcal{B}$. We provide detailed computations for interesting cases of chordal nestohedra, including permutohedra, associahedra, stellohedra,...

9. Partitions of ordered partitions and Bott manifolds

Junho Jeong (Chungbuk National University)

29/08/2024, 15:30

Bott manifolds are smooth projective toric varieties providing interesting avenues among topology, geometry, representation theory, and combinatorics. They are used to understand the geometric structure of Bott-Samelson-Demazure-Hansen (BSDH) varieties, which provide desingularizations of Schubert varieties. However, not all Bott manifolds originate from BSDH varieties. Those that do are...

5. Homotopy Types of Vietoris-Rips Complexes and Their Connection to Hyperconvexity

Sunhyuk Lim (Sungkyunkwan University)

29/08/2024, 16:30

Contributed talk

The Vietoris-Rips complex, originally introduced by Leopold Vietoris in the early 1900s to develop a homology theory for metric spaces, has since found applications in various areas of mathematics. Eliyahu Rips and Mikhail Gromov further utilized it in their studies of hyperbolic groups. More recently, classifying the homotopy types of Vietoris-Rips complexes has become a significant problem...

6. On the extremal number of face-incidence graphs

Jisun Baek (Yonsei University)

29/08/2024, 17:00

Contributed talk

The $(k,r)$-incidence graph of a regular polytope $\mathcal{P}$ is the bipartite incidence graph between $k$-faces and $r$-faces of $\mathcal{P}$. We obtain a general upper bound and a corresponding supersaturation result for the extremal number of the $(k,r)$-incidence graph of any regular polytope.
This generalises recent results of Janzer and Sudakov, who obtained the same bound for...

13. Combinatorics of orthogonal polynomials on the unit circle

Minho Song (Sungkyunkwan University)

30/08/2024, 11:00

Contributed talk

Orthogonal polynomials on the unit circle (OPUC) are a family of polynomials orthogonal with respect to integration on the unit circle in the complex plane. The values of these integrals can be obtained by calculating moments. Numerous combinatorial studies have explored the moments of various types of orthogonal polynomials, including classical orthogonal polynomials, Laurent biorthogonal...

4. Two ways to generalize matroids with coefficients

Donggyu Kim (KAIST & IBS DIMAG)

30/08/2024, 14:30

Dress (1986) introduced matroids with coefficients offering a unified approach to ordinary matroids, representations of matroids over fields, and oriented matroids. Baker and Bowler (2019) extended this theory, whose result includes a partial field representation by Semple and Whittle (1996).

I will present two generalizations of matroids with coefficients. One is about skew-symmetric...