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.

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

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