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arXiv · 1702.00521

Steiner triple systems with high chromatic index

Darryn Bryant·Charles Colbourn·Daniel Horsley·Ian M. Wanless
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Abstract

It is conjectured that every Steiner triple system of order $v \neq 7$ has chromatic index at most $(v+3)/2$ when $v \equiv 3 \pmod{6}$ and at most $(v+5)/2$ when $v \equiv 1 \pmod{6}$. Herein, we construct a Steiner triple system of order $v$ with chromatic index at least $(v+3)/2$ for each integer $v \equiv 3 \pmod{6}$ such that $v \geq 15$, with four possible exceptions. We further show that the maximum number of disjoint parallel classes in the systems constructed is sublinear in $v$. Finally, we establish for each order $v \equiv 15 \pmod{18}$ that there are at least $v^{v^2(1/6+o(1))}$ non-isomorphic Steiner triple systems with chromatic index at least $(v+3)/2$ and that some of these systems are cyclic.

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Darryn Bryant, Charles Colbourn, Daniel Horsley, Ian M. Wanless. 2017-02-02. Steiner triple systems with high chromatic index. https://doi.org/10.1137/17m1114338

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