Constant congestion linkages in polynomially strong digraphs in polynomial time

TL;DR

This paper shows how to build in polynomial time a bramble of size £k and congestion $8 assuming that a large obstruction to directed treewidth (namely, a path system) is given and shows that there is a polynomial function $g(k)$ such that every g(k)-strong digraph is $(k,8)$-linked.

Abstract

Given integers $k,c > 0$, we say that a digraph $D$ is $(k,c)$-linked if for every pair of ordered sets $\{s_1, \ldots, s_k\}$ and $\{t_1, \ldots, t_k\}$ of vertices of $D$, there are $P_1, \ldots, P_k$ such that for $i \in [k]$ each $P_i$ is a path from $s_i$ to $t_i$ and every vertex of $D$ appears in at most $c$ of those paths. Thomassen [Combinatorica, 1991] showed that for every fixed $k \geq 2$ there is no integer $p$ such that every $p$-strong digraph is $(k,1)$-linked. Edwards et al. [ESA, 2017] showed that every digraph $D$ with directed treewidth at least some function $f(k)$ contains a large bramble of congestion $2$ and that every $(36k^3 + 2k)$-strong digraph containing a bramble of congestion $2$ and size roughly $188k^3$ is $(k,2)$-linked. Since the directed treewidth of a digraph has to be at least its strong connectivity, this implies that there is a function $L(k)$ such that every $L(k)$-strong digraph is $(k,2)$-linked. This result was improved by Campos et al. [ESA, 2023], who showed that any $k$-strong digraph containing a bramble of size at least $2k(c\cdot k -c + 2) + c(k-1)$ and congestion $c$ is $(k,c)$-linked. Regarding the bramble, although the given bound on $f(k)$ is very large, Masařík et al. [SIDMA, 2022] showed that directed treewidth $\mathcal{O}(k^{48}\log^{13} k)$ suffices if the congestion is relaxed to $8$. We first show how to drop the dependence on $c$, for even $c$, on the size of the bramble that is needed in the work of Campos et al. [ESA, 2023]. Then, by making two local changes in the proof of Masařík et al. [SIDMA, 2022] we show how to build in polynomial time a bramble of size $k$ and congestion $8$ assuming that a large obstruction to directed treewidth (namely, a path system) is given. Applying these results, we show that there is a polynomial function $g(k)$ such that every $g(k)$-strong digraph is $(k,8)$-linked.

Figures (1)

• Figure 1: An $(a,b)$-path system with $a =3$. A thick edge denotes a linkage of size $b$ from a set $A_i^{\text{out}}$ to a set $A_j^{\text{in}}$, with $i \neq j$.

Theorems & Definitions (25)

• Theorem 1: Menger's Theorem Menger1927
• Definition 2: Brambles in digraphs
• Proposition 3: Campos et al. Campos2022, bramble version
• Definition 4: Linkages
• Definition 5: Well-linked sets
• Definition 6: Path system
• Proposition 7
• Proposition 8: Masařı́k et al. Masarik2022
• Theorem 9
• Proposition 10