Solutions to the linkage conjecture in tournaments

TL;DR

This paper disproves a conjecture that high connectivity and degree conditions guarantee k-linkedness in tournaments, by constructing counterexamples, and establishes new bounds for k-linkedness in semicomplete digraphs.

Contribution

It provides counterexamples to Pokrovskiy's conjecture and proves that high connectivity and degree conditions ensure k-linkedness in semicomplete digraphs.

Findings

Counterexamples disprove the linkage conjecture in tournaments.

High connectivity and degree conditions do not always imply k-linkedness.

A new degree bound ensures k-linkedness in semicomplete digraphs.

Abstract

A digraph $D$ is $k$-linked if for every $2k$-tuple $ x_1,\ldots , x_k, y_1, \ldots , y_k$ of distinct vertices in $D$, there exist $k$ pairwise vertex-disjoint paths $P_1,\ldots, P_k$ such that $P_i$ starts at $x_i$ and ends at $y_i$, $i\in [k]$. In 2015, Pokrovskiy conjectured that there exists a function $g(k)$ such that every $2k$-connected tournament with minimum in-degree and minimum out-degree at least $g(k)$ is $k$-linked in [J. Comb. Theory, Ser. B 115 (2015) 339--347]. In this paper, we disprove this conjecture by constructing a family of counterexamples. The counterexamples also provide a negative answer to the question raised by Gir\~{a}o, Popielarz, Snyder in [Combinatorica 41 (2021) 815--837]. Further, we prove that every $(2k+1)$-connected semicomplete digraph $D$ with minimum out-degree at least $ 10^7k^{4}$ is $k$-linked, which refines and generalizes the early result of Gir\~{a}o, Popielarz, Snyder.