Forbes, A.D.; Grannell, M.J. and Griggs, T.S.
Independent sets in Steiner triple systems.
Ars Combinatoria, 72 (LXXII)
A set of points in a Steiner triple system is said to be independent if no three of these points occur in the same block. In this paper we derive for each a closed formula for the number of independent sets of cardinality in an . We use the formula to prove that every STS(21) has an independent set of cardinality eight and is as a consequence 4-colourable.
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