A330057 Number of set-systems covering n vertices with no singletons or endpoints.

1, 0, 0, 5, 1703, 66954642, 144115175199102143, 1329227995784915808340204290157341181, 226156424291633194186662080095093568664788471116325389572604136316742486364

Offset

0,4

Comments

A set-system is a finite set of finite nonempty set of positive integers. A singleton is an edge of size 1. An endpoint is a vertex appearing only once (degree 1).

Links

Andrew Howroyd, Table of n, a(n) for n = 0..11

Wikipedia, Degree (graph theory)

Formula

Binomial transform is A330056.

Example

The a(3) = 5 set-systems:
  {{1,2},{1,3},{2,3}}
  {{1,2},{1,3},{1,2,3}}
  {{1,2},{2,3},{1,2,3}}
  {{1,3},{2,3},{1,2,3}}
  {{1,2},{1,3},{2,3},{1,2,3}}

Mathematica

Table[Length[Select[Subsets[Subsets[Range[n], {2, n}]], Union@@#==Range[n]&&Min@@Length/@Split[Sort[Join@@#]]>1&]], {n, 0, 4}]

Prog

(PARI) \\ here b(n) is A330056(n).
AS2(n, k) = {sum(i=0, min(n, k), (-1)^i * binomial(n, i) * stirling(n-i, k-i, 2) )}
b(n) = {sum(k=0, n, (-1)^k*binomial(n, k)*2^(2^(n-k)-(n-k)-1) * sum(j=0, k\2, sum(i=0, k-2*j, binomial(k, i) * AS2(k-i, j) * (2^(n-k)-1)^i * 2^(j*(n-k)) )))}
a(n) = {sum(k=0, n, (-1)^k*binomial(n, k)*b(n-k))} \\ Andrew Howroyd, Jan 16 2023

Crossrefs

The version for non-isomorphic set-systems is A330055 (by weight).

The non-covering version is A330056.

Set-systems with no singletons are A016031.

Set-systems with no endpoints are A330059.

Non-isomorphic set-systems with no singletons are A306005 (by weight).

Non-isomorphic set-systems with no endpoints are A330054 (by weight).

Non-isomorphic set-systems counted by vertices are A000612.

Non-isomorphic set-systems counted by weight are A283877.

Cf. A007716, A055621, A302545, A317533, A317794, A319559, A320665, A321405, A330052, A330058.

Sequence in context: A198246 A122465 A203683 * A324265 A003733 A364638

Adjacent sequences: A330054 A330055 A330056 * A330058 A330059 A330060

Keyword

nonn

Author

Gus Wiseman, Nov 30 2019

Extensions

Terms a(5) and beyond from Andrew Howroyd, Jan 16 2023

Status

approved