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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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6
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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
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,2
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COMMENTS
| 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
| For labeled case see Goulden I. P., Jackson D. M., Combinatorial Enumeration, John Wiley and Sons, New York, 1983.
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LINKS
| Vladeta Jovovic, Formula
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CROSSREFS
| Cf. A050535, A050913, A058783-A058785, A082789, A082790.
Sequence in context: A086836 A074513 A007871 * A199746 A080599 A120575
Adjacent sequences: A058787 A058788 A058789 * A058791 A058792 A058793
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KEYWORD
| nonn
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AUTHOR
| Vladeta Jovovic (vladeta(AT)eunet.rs), Nov 30 2000
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EXTENSIONS
| More terms from T. Forbes (anthony.d.forbes(AT)googlemail.com), May 24, 2003
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