arXiv subjects

Qiqin Xie

Publications and source records attributed to Qiqin Xie.

5 recordsLinked to original sources

arXiv2024

A half-integral Erdős-Pósa theorem for directed odd cycles

We prove that there exists a function $f:\mathbb{N}\rightarrow \mathbb{R}$ such that every directed graph $G$ contains either $k$ directed odd cycles where every vertex of $G$ is contained in at most two of them, or a set of at most $f(k)$ vertices meeting all directed odd cycles. We also give a polynomial-time algorithm for fixed $k$ which outputs one of the two outcomes. Using this algorithmic result, we give a polynomial-time algorithm for fixed $k$ to decide whether such $k$ directed odd cycles exist, or there are no $k$ vertex-disjoint directed odd cycles. This extends the half-integral Erdős-Pósa theorem for undirected odd cycles by Reed [Combinatorica 1999] to directed graphs.

math.CO↗

arXiv2023

A Ramsey Type problem for highly connected subgraphs

Bollobás and Gyárfás conjectured that for any $k, n \in \mathbb{Z}^+$ with $n > 4(k-1)$, every 2-edge-coloring of the complete graph on $n$ vertices leads to a $k$-connected monochromatic subgraph with at least $n-2k+2$ vertices. We find a counterexample with $n = \lfloor 5k-2.5-\sqrt{8k-\frac{31}{4}} \rfloor$, thus disproving the conjecture, and we show the conclusion holds for $n > 5k-2.5-\sqrt{8k-\frac{31}{4}}$ when $k \ge 16$.

math.CO↗

arXiv2020

4-Separations in Hajós Graphs

As a natural extension of the Four Color Theorem, Hajós conjectured that graphs containing no $K_5$-subdivision are 4-colorable. Any possible counterexample to this conjecture with minimum number of vertices is called a {\it Hajós graph}. Previous results show that Hajós graphs are 4-connected but not 5-connected. A $k$-separation in a graph $G$ is a pair $(G_1,G_2)$ of edge-disjoint subgraphs of $G$ such that $|V(G_1\cap G_2)|=k$, $G=G_1\cup G_2$, and $G_i\not\subseteq G_{3-i}$ for $i=1,2$. In this paper, we show that Hajós graphs do not admit a 4-separation $(G_1,G_2)$ such that $|V(G_1)|\ge 6$ and $G_1$ can be drawn in the plane with no edge crossings and all vertices in $V(G_1\cap G_2)$ incident with a common face. This is a step in our attempt to reduce Hajós' conjecture to the Four Color Theorem.

math.CO↗

arXiv2019

Wheels in planar graphs and Hajós graphs

It was conjectured by Hajós that graphs containing no $K_5$-subdivision are 4-colorable. Previous results show that any possible minimum counterexample to Hajós' conjecture, called Hajós graph, is 4-connected but not 5-connected. In this paper, we show that if a Hajós graph admits a 4-cut or 5-cut with a planar side then the planar side must be small or contains a special wheel. This is a step in our effort to reduce Hajós' conjecture to the Four Color Theorem.

math.CO↗

arXiv2016

Induced Forests in Bipartite Planar Graphs

Akiyama and Watanabe conjectured that every simple planar bipartite graph on $n$ vertices contains an induced forest on at least $5n/8$ vertices. We apply the discharging method to show that every simple bipartite planar graph on $n$ vertices contains an induced forest on at least $\lceil (4n+3)/7 \rceil$ vertices.

math.CO↗