A304998 Number of unlabeled antichains of finite sets spanning n vertices without singletons.

1, 0, 1, 3, 15, 160, 15963, 489980652

Offset: 0

Author

Gus Wiseman, May 23 2018

Keywords

• nonn

Examples

			Non-isomorphic representatives of the a(4) = 15 antichains:
  {{1,2,3,4}}
  {{1,2},{3,4}}
  {{1,4},{2,3,4}}
  {{1,3,4},{2,3,4}}
  {{1,2},{1,3,4},{2,3,4}}
  {{1,3},{1,4},{2,3,4}}
  {{1,3},{2,4},{3,4}}
  {{1,4},{2,4},{3,4}}
  {{1,2,4},{1,3,4},{2,3,4}}
  {{1,2},{1,3},{1,4},{2,3,4}}
  {{1,2},{1,3},{2,4},{3,4}}
  {{1,4},{2,3},{2,4},{3,4}}
  {{1,2,3},{1,2,4},{1,3,4},{2,3,4}}
  {{1,3},{1,4},{2,3},{2,4},{3,4}}
  {{1,2},{1,3},{1,4},{2,3},{2,4},{3,4}}
		

Crossrefs

Cf. A006126, A261005, A304996, A304997, A304998, A304999, A305000, A305001.

Formula

a(n > 0) = A261005(n) - A261005(n - 1).