Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userStack-number is not bounded by queue-number
88
0
0.0
(
0
)
Added by
David Wood
Publication date
2020
fields
Informatics Engineering
and research's language is
English
Ask ChatGPT about the research
No Arabic abstract
We describe a family of graphs with queue-number at most 4 but unbounded stack-number. This resolves open problems of Heath, Leighton and Rosenberg (1992) and Blankenship and Oporowski (1999).
rate research
Read More
Let $P subseteq mathbb{R}^2$ be a set of points and $T$ be a spanning tree of $P$. The emph{stabbing number} of $T$ is the maximum number of intersections any line in the plane determines with the edges of $T$. The emph{tree stabbing number} of $P$ is the minimum stabbing number of any spanning tree of $P$. We prove that the tree stabbing number is not a monotone parameter, i.e., there exist point sets $P subsetneq P$ such that treestab{$P$} $>$ treestab{$P$}, answering a question by Eppstein cite[Open Problem~17.5]{eppstein_2018}.
We focus on counting the number of labeled graphs on $n$ vertices and treewidth at most $k$ (or equivalently, the number of labeled partial $k$-trees), which we denote by $T_{n,k}$. So far, only the particular cases $T_{n,1}$ and $T_{n,2}$ had been studied. We show that $$ left(c cdot frac{kcdot 2^k cdot n}{log k} right)^n cdot 2^{-frac{k(k+3)}{2}} cdot k^{-2k-2} leq T_{n,k} leq left(k cdot 2^k cdot nright)^n cdot 2^{-frac{k(k+1)}{2}} cdot k^{-k}, $$ for $k > 1$ and some explicit absolute constant $c > 0$. The upper bound is an immediate consequence of the well-known number of labeled $k$-trees, while the lower bound is obtained from an explicit algorithmic construction. It follows from this construction that both bounds also apply to graphs of pathwidth and proper-pathwidth at most $k$.
In this short note, we show that for any $epsilon >0$ and $k<n^{0.5-epsilon}$ the choice number of the Kneser graph $KG_{n,k}$ is $Theta (nlog n)$.
Let $G=(V,E)$ be a graph and $n$ a positive integer. Let $I_n(G)$ be the abstract simplicial complex whose simplices are the subsets of $V$ that do not contain an independent set of size $n$ in $G$. We study the collapsibility numbers of the complexes $I_n(G)$ for various classes of graphs, focusing on the class of graphs with maximum degree bounded by $Delta$. As an application, we obtain the following result: Let $G$ be a claw-free graph with maximum degree at most $Delta$. Then, every collection of $leftlfloorleft(frac{Delta}{2}+1right)(n-1)rightrfloor+1$ independent sets in $G$ has a rainbow independent set of size $n$.
In this paper, we give bounds on the dichromatic number $vec{chi}(Sigma)$ of a surface $Sigma$, which is the maximum dichromatic number of an oriented graph embeddable on $Sigma$. We determine the asymptotic behaviour of $vec{chi}(Sigma)$ by showing that there exist constants $a_1$ and $a_2$ such that, $ a_1frac{sqrt{-c}}{log(-c)} leq vec{chi}(Sigma) leq a_2 frac{sqrt{-c}}{log(-c)} $ for every surface $Sigma$ with Euler characteristic $cleq -2$. We then give more explicit bounds for some surfaces with high Euler characteristic. In particular, we show that the dichromatic numbers of the projective plane $mathbb{N}_1$, the Klein bottle $mathbb{N}_2$, the torus $mathbb{S}_1$, and Dycks surface $mathbb{N}_3$ are all equal to $3$, and that the dichromatic numbers of the $5$-torus $mathbb{S}_5$ and the $10$-cross surface $mathbb{N}_{10}$ are equal to $4$. We also consider the complexity of deciding whether a given digraph or oriented graph embedabble in a fixed surface is $k$-dicolourable. In particular, we show that for any surface, deciding whether a digraph embeddable on this surface is $2$-dicolourable is NP-complete, and that deciding whether a planar oriented graph is $2$-dicolourable is NP-complete unless all planar oriented graphs are $2$-dicolourable (which was conjectured by Neumann-Lara).
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
