Duchet, Pierre (1995), “Hypergraphs”, in Graham, R. L.; Grötschel, M.; Lovász, L., Handbook of combinatorics, Vol. 1, 2, Amsterdam: Elsevier, pp. 381-432, MR1373663. See in particular Section 2.5, "Helly Property", pp. 393–394.
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Duchet, Pierre (1995), “Hypergraphs”, in Graham, R. L.; Grötschel, M.; Lovász, L., Handbook of combinatorics, Vol. 1, 2, Amsterdam: Elsevier, pp. 381-432, MR1373663. See in particular Section 2.5, "Helly Property", pp. 393–394.