

A058790


Number of covers of an unlabeled nset 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
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OFFSET

1,2


COMMENTS

Cover may include multiple occurrences of a subset. Also nrowed 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

Table of n, a(n) for n=1..21.
Vladeta Jovovic, Formula


CROSSREFS

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


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



