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 A056164 Number of ordered antichain covers of an unlabeled n-set; labeled T_1-hypergraphs (without empty hyperedges) with n hyperedges. 0
 1, 2, 6, 109, 191177 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS A T_1-hypergraph is a hypergraph (not necessarily without empty hyperedges or multiple hyperedges) which for every ordered pair of distinct nodes has a hyperedge containing one but not the other node. REFERENCES V. Jovovic and G. Kilibarda, On the number of Boolean functions in the Post classes F^{mu}_8, Diskretnaya Matematika, 11 (1999), no. 4, 127-138 (translated in Discrete Mathematics and Applications, 9, (1999), no. 6) V. Jovovic and G. Kilibarda, On enumeration of the class of all monotone Boolean functions, in preparation. LINKS Table of n, a(n) for n=1..5. K. S. Brown, Dedekind's problem Eric Weisstein's World of Mathematics, Antichain covers FORMULA a(n)=Sum_{k=1..C(n, floor(n/2))}b(k, n) where b(k, n) is the number of k-element ordered antichains covers of an unlabeled n-set. EXAMPLE There are 6 ordered antichain covers on an unlabeled 3-set: ({1,2,3}), ({1},{2,3}), ({2,3},{1}), ({1,2},{1,3}), ({1},{2},{3}), ({1,2},{1,3},{2,3}). a(3)=1+3+2=6; a(4)=1+6+17+25+30+30=109; a(5)=1+10+71+429+2176+8310+20580+38640+60480+60480=191177. CROSSREFS Cf. A056074, A056090, A056093, A000372, A056005, A056069-A056071, A056073, A056046-A056049, A056052, A056101, A056104, A051112-A051118. Sequence in context: A059088 A216151 A057771 * A156500 A264889 A365669 Adjacent sequences: A056161 A056162 A056163 * A056165 A056166 A056167 KEYWORD hard,more,nonn AUTHOR Vladeta Jovovic, Goran Kilibarda, Jul 31 2000 STATUS approved

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Last modified April 13 20:16 EDT 2024. Contains 371645 sequences. (Running on oeis4.)