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A058790
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Number of covers of an unlabeled n-set such that every point of the set is covered by exactly 3 subsets of the cover and that intersection of every 3 subsets of the cover contains at most one point.
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7
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1, 3, 12, 66, 445, 4279, 53340, 846254, 16333946, 371976963, 9763321109, 290473143807, 9674133467729, 357177322891321, 14503958827502886, 643502334799711633, 31018731336031551119, 1616523352051185316626, 90689288905913623412837, 5456178840303106057314759, 350830170593891706361540379
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OFFSET
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1,2
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COMMENTS
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Cover may include multiple occurrences of a subset. Also n-rowed binary matrices with distinct rows and all row sums 3.
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REFERENCES
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For labeled case see Goulden I. P., Jackson D. M., Combinatorial Enumeration, John Wiley and Sons, New York, 1983.
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LINKS
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Table of n, a(n) for n=1..21.
Vladeta Jovovic, Formula
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CROSSREFS
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Row n=3 of A331508.
Cf. A050535, A050913, A058783, A058784, A058785, A082789, A082790.
Sequence in context: A007871 A214565 A267323 * A199746 A293302 A248871
Adjacent sequences: A058787 A058788 A058789 * A058791 A058792 A058793
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KEYWORD
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nonn
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AUTHOR
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Vladeta Jovovic, Nov 30 2000
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EXTENSIONS
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More terms from T. Forbes (anthony.d.forbes(AT)googlemail.com), May 24, 2003
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STATUS
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approved
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