A326911 BII-numbers of set-systems with empty intersection.

0, 3, 7, 9, 10, 11, 12, 13, 14, 15, 18, 19, 22, 23, 25, 26, 27, 28, 29, 30, 31, 33, 35, 37, 39, 41, 42, 43, 44, 45, 46, 47, 49, 50, 51, 52, 53, 54, 55, 57, 58, 59, 60, 61, 62, 63, 67, 71, 73, 74, 75, 76, 77, 78, 79, 82, 83, 86, 87, 89, 90, 91, 92, 93, 94, 95

Offset: 1

Author

Gus Wiseman, Aug 04 2019

Keywords

• nonn

Comments

A binary index of n is any position of a 1 in its reversed binary expansion. The binary indices of n are row n of A048793. We define the set-system with BII-number n to be obtained by taking the binary indices of each binary index of n. Every finite set of finite nonempty sets has a different BII-number. For example, 18 has reversed binary expansion (0,1,0,0,1), and since the binary indices of 2 and 5 are {2} and {1,3} respectively, the BII-number of {{2},{1,3}} is 18. Elements of a set-system are sometimes called edges.

Examples

			The sequence of all set-systems with empty intersection together with their BII-numbers begins:
   0: {}
   3: {{1},{2}}
   7: {{1},{2},{1,2}}
   9: {{1},{3}}
  10: {{2},{3}}
  11: {{1},{2},{3}}
  12: {{1,2},{3}}
  13: {{1},{1,2},{3}}
  14: {{2},{1,2},{3}}
  15: {{1},{2},{1,2},{3}}
  18: {{2},{1,3}}
  19: {{1},{2},{1,3}}
  22: {{2},{1,2},{1,3}}
  23: {{1},{2},{1,2},{1,3}}
  25: {{1},{3},{1,3}}
  26: {{2},{3},{1,3}}
  27: {{1},{2},{3},{1,3}}
  28: {{1,2},{3},{1,3}}
  29: {{1},{1,2},{3},{1,3}}
  30: {{2},{1,2},{3},{1,3}}
		

Crossrefs

Cf. A048793, A051185, A305843, A317752, A317755, A317757, A319077, A326031, A326910, A326912.

Programs

Mathematica

bpe[n_]:=Join@@Position[Reverse[IntegerDigits[n,2]],1];
Select[Range[0,100],#==0||Intersection@@bpe/@bpe[#]=={}&]