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Highly linked tournaments. (English) Zbl 1319.05063
Summary: 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 B. Bollobás and A. Thomason [Combinatorica 16, No. 3, 313–320 (1996; Zbl 0870.05044)] says that every 22$$k$$-connected (undirected) graph is $$k$$-linked. It is desirable to obtain analogues for directed graphs as well. Although C. Thomassen [“Note on highly connected non-2-linked digraphs”, Combinatorica 11, No. 3, 393–395 (1991)] showed that the Bollobás-Thomason theorem does not hold for general directed graphs, he [“Connectivity in tournaments”, in: B. Bollobás (ed.), Graph theory and combinatorics. Proceedings of the Cambridge combinatorial conference, in honour of Paul Erdős. London: Academic Press. 305–313 (1984)] 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 D. Kühn et al. [Proc. Lond. Math. Soc. (3) 109, No. 3, 733–762 (2014; Zbl 1302.05069)], who also conjectured that a linear bound should hold. We prove this conjecture, by showing that every strongly $$452k$$-connected tournament is $$k$$-linked.

MSC:
 05C20 Directed graphs (digraphs), tournaments 05C40 Connectivity
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References:
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