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A058790
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.
7
1, 3, 12, 66, 445, 4279, 53340, 846254, 16333946, 371976963, 9763321109, 290473143807, 9674133467729, 357177322891321, 14503958827502886, 643502334799711633, 31018731336031551119, 1616523352051185316626, 90689288905913623412837, 5456178840303106057314759, 350830170593891706361540379
OFFSET
1,2
COMMENTS
Cover may include multiple occurrences of a subset. Also n-rowed binary matrices with distinct rows and all row sums 3.
REFERENCES
For labeled case see Goulden I. P., Jackson D. M., Combinatorial Enumeration, John Wiley and Sons, New York, 1983.
LINKS
CROSSREFS
KEYWORD
nonn
AUTHOR
Vladeta Jovovic, Nov 30 2000
EXTENSIONS
More terms from T. Forbes (anthony.d.forbes(AT)googlemail.com), May 24, 2003
STATUS
approved