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Seonghyuk Im

Publications and source records attributed to Seonghyuk Im.

22 records · Page 2Linked to original sources

A bandwidth theorem for graph transversals

Given a collection $\mathcal{G}=(G_1,\dots, G_h)$ of graphs on the same vertex set $V$ of size $n$, an $h$-edge graph $H$ on the vertex set $V$ is a $\mathcal{G}$-transversal if there exists a bijection $λ: E(H) \rightarrow [h]$ such that $e\in E(G_{λ(e)})$ for each $e\in E(H)$. The conditions on the minimum degree $δ(\mathcal{G})=\min_{i\in[h]}\{ δ(G_i)\}$ for finding a spanning $\mathcal{G}$-transversal isomorphic to a graph $H$ have been actively studied when $H$ is a Hamilton cycle, an $F$-factor, a spanning tree with maximum degree $o(n/\log n)$ and a power of a Hamilton cycle, etc. In this paper, we determined the asymptotically tight threshold on $δ(\mathcal{G})$ for finding a $\mathcal{G}$-transversal isomorphic to $H$ when $H$ is a general $n$-vertex graph with bounded maximum degree and $o(n)$-bandwidth. This provides a transversal generalization of the celebrated Bandwidth theorem by Böttcher, Schacht and Taraz.

math.CO↗

A proof of the Elliott-Rödl conjecture on hypertrees in Steiner triple systems

Hypertrees are linear hypergraphs where every two vertices are connected by a unique path. Elliott and Rödl conjectured that for any given $μ>0$, there exists $n_0$ such that the following holds. Every $n$-vertex Steiner triple system contains all hypertrees with at most $(1-μ)n$ vertices whenever $n\geq n_0$. We prove this conjecture.

math.CO↗

The proper conflict-free $k$-coloring problem and the odd $k$-coloring problem are NP-complete on bipartite graphs

A proper coloring of a graph is \emph{proper conflict-free} if every non-isolated vertex $v$ has a neighbor whose color is unique in the neighborhood of $v$. A proper coloring of a graph is \emph{odd} if for every non-isolated vertex $v$, there is a color appearing an odd number of times in the neighborhood of $v$. For an integer $k$, the \textsc{PCF $k$-Coloring} problem asks whether an input graph admits a proper conflict-free $k$-coloring and the \textsc{Odd $k$-Coloring} asks whether an input graph admits an odd $k$-coloring. We show that for every integer $k\geq3$, both problems are NP-complete, even if the input graph is bipartite. Furthermore, we show that the \textsc{PCF $4$-Coloring} problem is NP-complete when the input graph is planar.

cs.CC↗

Complexity of Partitioning Hypergraphs

For a given $π=(π_0, π_1,..., π_k) \in \{0, 1, *\}^{k+1}$, we want to determine whether an input $k$-uniform hypergraph $G=(V, E)$ has a partition $(V_1, V_2)$ of the vertex set so that for all $X \subseteq V$ of size $k$, $X \in E$ if $π_{|X\cap V_1|}=1$ and $X \notin E$ if $π_{|X\cap V_1|}=0$. We prove that this problem is either polynomial-time solvable or NP-complete depending on $π$ when $k=3$ or $4$. We also extend this result into $k$-uniform hypergraphs for $k \geq 5$.

cs.CC↗