OFFSET
0,5
LINKS
Andrew Howroyd, Table of n, a(n) for n = 0..1000
FORMULA
a(n) = binomial(n,2) for n >= 5. - Gus Wiseman, Jan 16 2019
Binomial transform is A289837. - Gus Wiseman, Jan 16 2019
a(n) = A000217(n-1) for n >= 5. - Alois P. Heinz, Jan 24 2019
E.g.f.: 1 - x^2/2 - x^3/3 + 5*x^4/24 + x^2*exp(x)/2. - Andrew Howroyd, Aug 18 2019
EXAMPLE
The a(4) = 11 hypergraphs:
{{1,2,3},{1,2,4}}
{{1,2,3},{1,3,4}}
{{1,2,3},{2,3,4}}
{{1,2,4},{1,3,4}}
{{1,2,4},{2,3,4}}
{{1,3,4},{2,3,4}}
{{1,2,3},{1,2,4},{1,3,4}}
{{1,2,3},{1,2,4},{2,3,4}}
{{1,2,3},{1,3,4},{2,3,4}}
{{1,2,4},{1,3,4},{2,3,4}}
{{1,2,3},{1,2,4},{1,3,4},{2,3,4}}
MATHEMATICA
stableSets[u_, Q_]:=If[Length[u]===0, {{}}, With[{w=First[u]}, Join[stableSets[DeleteCases[u, w], Q], Prepend[#, w]&/@stableSets[DeleteCases[u, r_/; r===w||Q[r, w]||Q[w, r]], Q]]]];
Table[Length[Select[stableSets[Subsets[Range[n], {3}], Length[Intersection[#1, #2]]<=1&], Union@@#==Range[n]&]], {n, 10}]
PROG
(PARI) seq(n)={Vec(serlaplace(1 - x^2/2 - x^3/3 + 5*x^4/24 + x^2*exp(x + O(x^(n-1)))/2))} \\ Andrew Howroyd, Aug 18 2019
CROSSREFS
KEYWORD
nonn
AUTHOR
Gus Wiseman, Jan 10 2019
STATUS
approved