A305001 -id:A305001 - cp's OEIS frontend

A305055 Numbers n such that the z-density of the integer partition with Heinz number n is 0.

1, 169, 481, 507, 793, 841, 845, 1157, 1183, 1369, 1443, 1469, 1521, 1849, 1963, 2059, 2209, 2257, 2353, 2379, 2405, 2523, 2535, 2899, 3211, 3263, 3277, 3293, 3367, 3471, 3549, 3653, 3721, 3887, 3965, 4107, 4121, 4181, 4225, 4329, 4394, 4407, 4563, 4601, 4667

Offset: 1

Gus Wiseman, May 24 2018

nonn

The Heinz number of an integer partition (y_1,...,y_k) is prime(y_1)*...*prime(y_k).

The z-density of a multiset S of positive integers is Sum_{s in S} (omega(s) - 1) - omega(lcm(S)) where omega = A001221 is number of distinct prime factors.

Cf. A001221, A030019, A048143, A056239, A112798, A290103, A302242, A303837, A304118, A304714, A304716, A305001, A305052, A305053, A305054.

zens[n_]:=If[n==1,0,Total@Cases[FactorInteger[n],{p_,k_}:>k*(PrimeNu[PrimePi[p]]-1)]-PrimeNu[LCM@@Cases[FactorInteger[n],{p_,k_}:>PrimePi[p]]]];
Select[Range[1000],zens[#]==0&]

A323820 Number of non-isomorphic connected set-systems covering n vertices with no singletons.

Original entry on oeis.org

1, 0, 1, 6, 171, 611846, 200253853704319, 263735716028826427334553304608242, 5609038300883759793482640992086670066496449147691597380632107520565546

Offset: 0

Gus Wiseman, Jan 30 2019

nonn

The labeled case is A323817.

			Non-isomorphic representatives 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}}

Cf. A000295, A003465, A016031, A048143, A055621, A293510, A305001, A317795 (not necessarily connected), A323817 (unlabeled case), A323819 (with singletons).

Inverse Euler transform of A317795.

A326375 Number of intersecting antichains of subsets of {1..n} with empty intersection (meaning there is no vertex in common to all the edges).

Original entry on oeis.org

2, 2, 2, 3, 29, 1961, 1379274, 229755337550, 423295079757497714060

Offset: 0

Gus Wiseman, Jul 03 2019

A set system (set of sets) is an antichain if no edge is a subset of any other, and is intersecting if no two edges are disjoint.

			The a(4) = 29 antichains:
  {}
  {{}}
  {{1,2},{1,3},{2,3}}
  {{1,2},{1,4},{2,4}}
  {{1,3},{1,4},{3,4}}
  {{2,3},{2,4},{3,4}}
  {{1,2},{1,3},{2,3,4}}
  {{1,2},{1,4},{2,3,4}}
  {{1,2},{2,3},{1,3,4}}
  {{1,2},{2,4},{1,3,4}}
  {{1,3},{1,4},{2,3,4}}
  {{1,3},{2,3},{1,2,4}}
  {{1,3},{3,4},{1,2,4}}
  {{1,4},{2,4},{1,2,3}}
  {{1,4},{3,4},{1,2,3}}
  {{2,3},{2,4},{1,3,4}}
  {{2,3},{3,4},{1,2,4}}
  {{2,4},{3,4},{1,2,3}}
  {{1,2},{1,3,4},{2,3,4}}
  {{1,3},{1,2,4},{2,3,4}}
  {{1,4},{1,2,3},{2,3,4}}
  {{2,3},{1,2,4},{1,3,4}}
  {{2,4},{1,2,3},{1,3,4}}
  {{3,4},{1,2,3},{1,2,4}}
  {{1,2},{1,3},{1,4},{2,3,4}}
  {{1,2},{2,3},{2,4},{1,3,4}}
  {{1,3},{2,3},{3,4},{1,2,4}}
  {{1,4},{2,4},{3,4},{1,2,3}}
  {{1,2,3},{1,2,4},{1,3,4},{2,3,4}}

The case without empty edges is A326366.
Intersecting antichains are A326372.
Antichains of nonempty sets with empty intersection are A006126 or A307249.
Cf. A001206, A007363, A014466, A051185, A058891, A305001, A305843, A305844, A318128, A318129, A326363, A326365, A326373.

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]],Or[Intersection[#1,#2]=={},SubsetQ[#1,#2]]&],#=={}||Intersection@@#=={}&]],{n,0,4}]

a(n) = A326366(n) + 1.

a(7)-a(8) from Andrew Howroyd, Aug 14 2019

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A305055 Numbers n such that the z-density of the integer partition with Heinz number n is 0.

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A323820 Number of non-isomorphic connected set-systems covering n vertices with no singletons.

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A326375 Number of intersecting antichains of subsets of {1..n} with empty intersection (meaning there is no vertex in common to all the edges).

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