무지개 집합 문제에서의 위상수학적 조합론

학위논문(박사)--서울대학교 대학원 :자연과학대학 수리과학부,2019. 8. 국웅.$\mathcal{F}=\{S_1,\ldots,S_m\}$를 $V$의 공집합이 아닌 부분 집합들의 모임이라 할 때, $\mathcal{F}$의 무지개 집합이란 공집합이 아니며 $S=\{s_{i_1},\ldots,s_{i_k}\} \subset V$와 같은 형태로 주어지는 것으로 다음 조건을 만족하는 것을 말한다. $1\leq i_1<\cdots<i_k \leq m$이고 $j \ne j$이면 $s_{i_j} \ne s_{i_j'}$를 만족하며 각 $j \in [m]$에 대해 $s_{i_j} \in S_{i_j}$이다. 특히 $k=m$인 경우, 즉 모든 $S_i$들이 표현되면, 무지개 집합 $S$를 $\mathcal{F}$의 완전 무지개 집합이라고 한다. 주어진 집합계가 특정 조건을 만족하는 무지개 집합을 가지기 위한 충분 조건을 찾는 문제는 홀의 결혼 정리에서 시작되어 최근까지도 조합수학에서 가장 대표적 문제 중 하나로 여겨져왔다. 이러한 방향으로의 문제를 무지개 집합 문제라고 부른다. 본 학위논문에서는 무지개 집합 문제와 관련하여 위상수학적 홀의 정리와 위상수학적 다색 헬리 정리를 소개하고, (하이퍼)그래프에서의 무지개 덮개와 무지개 독립 집합에 관한 결과들을 다루고자 한다.Let $\mathcal{F}=\{S_1,\ldots,S_m\}$ be a finite family of non-empty subsets on the ground set $V$. A rainbow set of $\mathcal{F}$ is a non-empty set of the form $S=\{s_{i_1},\ldots,s_{i_k}\} \subset V$ with $1 \leq i_1 < \cdots < i_k \leq m$ such that $s_{i_j} \neq s_{i_{j'}}$ for every $j \neq j'$ and $s_{i_j} \in S_{i_j}$ for each $j \in [k]$. If $k = m$, namely if all $S_i$ is represented, then the rainbow set $S$ is called a full rainbow set of $\mathcal{F}$. Originated from the celebrated Hall's marriage theorem, it has been one of the most fundamental questions in combinatorics and discrete mathematics to find sufficient conditions on set-systems to guarantee the existence of certain rainbow sets. We call problems in this direction the rainbow set problems. In this dissertation, we give an overview on two topological tools on rainbow set problems, Aharoni and Haxell's topological Hall theorem and Kalai and Meshulam's topological colorful Helly theorem, and present some results on and rainbow independent sets and rainbow covers in (hyper)graphs.Abstract i 1 Introduction 1 1.1 Topological Hall theorem . . . . . . . . . . . . . . . . . . . . . . 2 1.2 Topological colorful Helly theorem . . . . . . . . . . . . . . . . . 3 1.2.1 Collapsibility and Lerayness of simplicial complexes . . . 4 1.2.2 Nerve theorem and topological Helly theorem . . . . . . . 5 1.2.3 Topological colorful Helly theorem . . . . . . . . . . . . 6 1.3 Domination numbers and non-cover complexes of hypergraphs . . 7 1.3.1 Domination numbers of hypergraphs . . . . . . . . . . . . 10 1.3.2 Non-cover complexes of hypergraphs . . . . . . . . . . . . 10 1.4 Rainbow independent sets in graphs . . . . . . . . . . . . . . . . 12 1.5 Organization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 2 Collapsibility of non-cover complexes of graphs 16 2.1 The minimal exclusion sequences . . . . . . . . . . . . . . . . . . 16 2.2 Independent domination numbers and collapsibility numbers of non-cover complexes of graphs . . . . . . . . . . . . . . . . . . . 21 3 Domination numbers and non-cover complexes of hypergraphs 24 3.1 Proof of Theorem 1.3.4 . . . . . . . . . . . . . . . . . . . . . . . 25 3.1.1 Edge-annihilation . . . . . . . . . . . . . . . . . . . . . . 25 3.1.2 Non-cover complexes for hypergraphs . . . . . . . . . . . 27 3.2 Lerayness of non-cover complexes . . . . . . . . . . . . . . . . . 30 3.2.1 Total domination numbers . . . . . . . . . . . . . . . . . 30 3.2.2 Independent domination numbers . . . . . . . . . . . . . 33 3.2.3 Edgewise-domination numbers . . . . . . . . . . . . . . . 34 3.3 Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 3.3.1 Independent domination numbers of hypergraphs . . . . . 35 3.3.2 Independence complexes of hypergraphs . . . . . . . . . . 36 3.3.3 General position complexes . . . . . . . . . . . . . . . . . 37 3.3.4 Rainbow covers of hypergraphs . . . . . . . . . . . . . . 39 3.3.5 Collapsibility of non-cover complexes of hypergraphs . . . 40 4 Rainbow independent sets 42 4.1 Graphs avoiding certain induced subgraphs . . . . . . . . . . . . 42 4.1.1 Claw-free graphs . . . . . . . . . . . . . . . . . . . . . . 42 4.1.2 $\{C_4,C_5, . . . ,C_s\}$-free graphs . . . . . . . . . . . . . . . . . 44 4.1.3 Chordal graphs . . . . . . . . . . . . . . . . . . . . . . . 49 4.1.4 $K_r$-free graphs and $K^{−}_r$-free graphs . . . . . . . . . . . . . 50 4.2 $k$-colourable graphs . . . . . . . . . . . . . . . . . . . . . . . . . 54 4.3 Graphs with bounded degrees . . . . . . . . . . . . . . . . . . . . 55 4.3.1 The case $m < n$ . . . . . . . . . . . . . . . . . . . . . . . 56 4.4 A topological approach . . . . . . . . . . . . . . . . . . . . . . . 64 4.5 Concluding remark . . . . . . . . . . . . . . . . . . . . . . . . . 67 Abstract (in Korean) 69 Acknowledgement (in Korean) 70Docto

Further results on packing related parameters in graphs

Given a graph G = (V, E), a set B subset of V (G) is a packing in G if the closed neighborhoods of every pair of distinct vertices in B are pairwise disjoint. The packing number rho(G) of G is the maximum cardinality of a packing in G. Similarly, open packing sets and open packing number are defined for a graph G by using open neighborhoods instead of closed ones. We give several results concerning the (open) packing number of graphs in this paper. For instance, several bounds on these packing parameters along with some Nordhaus-Gaddum inequalities are given. We characterize all graphs with equal packing and independence numbers and give the characterization of all graphs for which the packing number is equal to the independence number minus one. In addition, due to the close connection between the open packing and total domination numbers, we prove a new upper bound on the total domination number gamma(t)(T) for a tree T of order n >= 2 improving the upper bound gamma(t)(T) <= (n + s)/2 given by Chellali and Haynes in 2004, in which s is the number of support vertices of T

Structure and properties of maximal outerplanar graphs.

Outerplanar graphs are planar graphs that have a plane embedding in which each vertex lies on the boundary of the exterior region. An outerplanar graph is maximal outerplanar if the graph obtained by adding an edge is not outerplanar. Maximal outerplanar graphs are also known as triangulations of polygons. The spine of a maximal outerplanar graph G is the dual graph of G without the vertex that corresponds to the exterior region. In this thesis we study metric properties involving geodesic intervals, geodetic sets, Steiner sets, different concepts of boundary, and also relationships between the independence numbers and domination numbers of maximal outerplanar graphs and their spines. In Chapter 2 we find an extension of a result by Beyer, et al. [3] that deals with Hamiltonian degree sequences in maximal outerplanar graphs. In Chapters 3 and 4 we give sharp bounds relating the independence number and domination number, respectively, of a maximal outerplanar graph to those of its spine. In Chapter 5 we discuss the boundary, contour, eccentricity, periphery, and extreme set of a graph. We give a characterization of the boundary of maximal outerplanar graphs that involves the degrees of vertices. We find properties that characterize the contour of a maximal outerplanar graph. The other main result of this chapter gives characterizations of graphs induced by the contour and by the periphery of a maximal outerplanar graph. In Chapter 6 we show that the generalized intervals in a maximal outerplanar graph are convex. We use this result to characterize geodetic sets in maximal outerplanar graphs. We show that every Steiner set in a maximal outerplanar graph is a geodetic set and also show some differences between these types of sets. We present sharp bounds for geodetic numbers and Steiner numbers of maximal outerplanar graphs

On the algorithmic complexity of twelve covering and independence parameters of graphs

The definitions of four previously studied parameters related to total coverings and total matchings of graphs can be restricted, thereby obtaining eight parameters related to covering and independence, each of which has been studied previously in some form. Here we survey briefly results concerning total coverings and total matchings of graphs, and consider the aforementioned 12 covering and independence parameters with regard to algorithmic complexity. We survey briefly known results for several graph classes, and obtain new NP-completeness results for the minimum total cover and maximum minimal total cover problems in planar graphs, the minimum maximal total matching problem in bipartite and chordal graphs, and the minimum independent dominating set problem in planar cubic graphs

A characterization of trees with equal 2-domination and 2-independence numbers

A set $S$ of vertices in a graph $G$ is a $2$-dominating set if every vertex of $G$ not in $S$ is adjacent to at least two vertices in $S$, and $S$ is a $2$-independent set if every vertex in $S$ is adjacent to at most one vertex of $S$. The $2$-domination number $\gamma_2(G)$ is the minimum cardinality of a $2$-dominating set in $G$, and the $2$-independence number $\alpha_2(G)$ is the maximum cardinality of a $2$-independent set in $G$. Chellali and Meddah [{\it Trees with equal $2$-domination and $2$-independence numbers,} Discussiones Mathematicae Graph Theory 32 (2012), 263--270] provided a constructive characterization of trees with equal $2$-domination and $2$-independence numbers. Their characterization is in terms of global properties of a tree, and involves properties of minimum $2$-dominating and maximum $2$-independent sets in the tree at each stage of the construction. We provide a constructive characterization that relies only on local properties of the tree at each stage of the construction.Comment: 17 pages, 4 figure