We give the first inductive characterization of planar graphs of treewidth $k$ for each $k \geq 3$. Our definition generalizes the well-known inductive definition of planar $3$-trees and the recent definition of so-called planar quasi-$4$-trees [Förster, 2024]. As a key step, which may be of independent interest, we show that every planar triangulation $G$ of treewidth $k$ admits a tree decomposition of width $k$ where each adhesion set induces a cycle in $G$. As an application of planar quasi-$4$-trees, we show that planar graphs of treewidth at most $4$ have vertex arboricity at most $2$. That is, the vertex-set of every planar graph of treewidth at most $4$ can be partitioned into two sets that each induce a forest.
Jan Diester, Kolja Kühn, Manuel Rapp, Samuel Schneider, Torsten Ueckerdt
原文摘要
We examine the computational complexity of well-known problems on subgraphs of the strong product of two paths. Namely, for Max Cut we show NP-hardness on induced subgraphs of $P \boxtimes P$ answering a question by Dvořák, Huynh, Joret, Liu, and Wood (2021). For 3-Edge Coloring we show NP-hardness on subgraphs of $P \boxtimes P$ and that it is equivalent to 3-Edge Coloring on planar graphs for induced subgraphs of $P \boxtimes P$. It is a long standing open question whether 3-Edge Coloring on planar graphs is NP-hard. As a second contribution, we consider graphs of the form $G \boxtimes P$ and show that Feedback Vertex Set on these graphs is FPT in the clique-width of $G$. Furthermore, we show that it can be solved in polynomial time if $G$ is chordal or co-comparable and that the computational complexity depends on the parity of $P$ if $G$ is bipartite. Namely, it is solvable in polynomial time if $P$ has an even number of vertices but is NP-complete otherwise. For Max Cut, we show that it can be solved in polynomial time if $G$ is bipartite.