Highly linked tournaments

Alexey Pokrovskiy

Published 2014-06-29Version 1

A (possibly directed) graph is $k$-linked if for any two disjoint sets of vertices $\{x_1, \dots, x_k\}$ and $\{y_1, \dots, y_k\}$ there are vertex disjoint paths $P_1, \dots, P_k$ such that $P_i$ goes from $x_i$ to $y_{i}$. A theorem of Bollob\'as and Thomason says that every $22k$-connected (undirected) graph is $k$-linked. It is desirable to obtain analogues for directed graphs as well. Although Thomassen showed that the Bollob\'as-Thomason Theorem does not hold for general directed graphs, he proved an analogue of the theorem for tournaments - there is a function $f(k)$ such that every strongly $f(k)$-connected tournament is $k$-linked. The bound on $f(k)$ was reduced to $O(k \log k)$ by K\"uhn, Lapinskas, Osthus, and Patel, who also conjectured that a linear bound should hold. We prove this conjecture, by showing that every strongly $452k$-connected tournament is $k$-linked.