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A305999 Number of unlabeled spanning intersecting set-systems on n vertices with no singletons. 7
1, 0, 1, 6, 76, 12916 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,4

COMMENTS

An intersecting set-system S is a finite set of finite nonempty sets (edges), any two of which have a nonempty intersection. S is spanning if every vertex is contained in some edge. A singleton is an edge containing only one vertex.

LINKS

Table of n, a(n) for n=0..5.

FORMULA

a(n) = A306001(n) - A306001(n-1) for n > 0. - Andrew Howroyd, Aug 12 2019

EXAMPLE

Non-isomorphic representative of the a(3) = 6 set-systems:

{{1,2,3}}

{{1,3},{2,3}}

{{2,3},{1,2,3}}

{{1,2},{1,3},{2,3}}

{{1,3},{2,3},{1,2,3}}

{{1,2},{1,3},{2,3},{1,2,3}}

CROSSREFS

Cf. A001206, A051185, A048143, A261006, A058891, A261005, A304998, A305854-A305857, A305935, A306000, A306001, A306008.

Sequence in context: A132613 A009763 A340886 * A274464 A028979 A082629

Adjacent sequences:  A305996 A305997 A305998 * A306000 A306001 A306002

KEYWORD

nonn,more

AUTHOR

Gus Wiseman, Jun 16 2018

EXTENSIONS

a(5) from Andrew Howroyd, Aug 12 2019

STATUS

approved

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Last modified December 1 16:44 EST 2021. Contains 349430 sequences. (Running on oeis4.)