Making a tournament \(k\)-arc-strong by reversing or deorienting arcs.

*(English)*Zbl 1035.05045Summary: We prove that every tournament \(T=(V,A)\) on \(n \geq 2k+1\) vertices can be made \(k\)-arc-strong by reversing no more than \(k(k+1)/2\) arcs. This is best possible as the transitive tournament needs this many arcs to be reversed. We show that the number of arcs we need to reverse in order to make a tournament \(k\)-arc-strong is closely related to the number of arcs we need to reverse just to achieve in- and out-degree at least \(k\). We also consider, for general digraphs, the operation of deorienting an arc which is not part of a 2-cycle. That is we replace an arc \(xy\) such that \(yx\) is not an arc by the 2-cycle \(xyx\). We prove that for every tournament \(T\) on at least \(2k+1\) vertices, the number of arcs we need to reverse in order to obtain a \(k\)-arc-strong tournament from \(T\) is equal to the number of arcs one needs to deorient in order to obtain a \(k\)-arc-strong digraph from \(T\). Finally, we discuss the relations of our results to related problems and conjectures.

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\textit{J. Bang-Jensen} and \textit{A. Yeo}, Discrete Appl. Math. 136, No. 2--3, 161--171 (2004; Zbl 1035.05045)

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##### References:

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