A326875 - OEIS

A326875

BII-numbers of set-systems that are closed under union.

0, 1, 2, 4, 5, 6, 7, 8, 16, 17, 24, 25, 32, 34, 40, 42, 64, 65, 66, 68, 69, 70, 71, 72, 76, 80, 81, 82, 84, 85, 86, 87, 88, 89, 92, 93, 96, 97, 98, 100, 101, 102, 103, 104, 106, 108, 110, 112, 113, 114, 116, 117, 118, 119, 120, 121, 122, 124, 125, 126, 127, 128

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OFFSET

1,3

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.

The enumeration of these set-systems by number of covered vertices is A102896.

LINKS

Table of n, a(n) for n=1..62.

EXAMPLE

The sequence of all set-systems that are closed under union together with their BII-numbers begins:

0: {}

1: {{1}}

2: {{2}}

4: {{1,2}}

5: {{1},{1,2}}

6: {{2},{1,2}}

7: {{1},{2},{1,2}}

8: {{3}}

16: {{1,3}}

17: {{1},{1,3}}

24: {{3},{1,3}}

25: {{1},{3},{1,3}}

32: {{2,3}}

34: {{2},{2,3}}

40: {{3},{2,3}}

42: {{2},{3},{2,3}}

64: {{1,2,3}}

65: {{1},{1,2,3}}

66: {{2},{1,2,3}}

68: {{1,2},{1,2,3}}

69: {{1},{1,2},{1,2,3}}

70: {{2},{1,2},{1,2,3}}

71: {{1},{2},{1,2},{1,2,3}}

72: {{3},{1,2,3}}

76: {{1,2},{3},{1,2,3}}

80: {{1,3},{1,2,3}}

81: {{1},{1,3},{1,2,3}}

82: {{2},{1,3},{1,2,3}}

84: {{1,2},{1,3},{1,2,3}}

85: {{1},{1,2},{1,3},{1,2,3}}

86: {{2},{1,2},{1,3},{1,2,3}}

MATHEMATICA

bpe[n_]:=Join@@Position[Reverse[IntegerDigits[n, 2]], 1];

Select[Range[0, 100], SubsetQ[bpe/@bpe[#], Union@@@Tuples[bpe/@bpe[#], 2]]&]

CROSSREFS

Cf. A006126, A048793, A102894, A102896, A102897, A326031, A326872, A326874, A326876, A326880.

Sequence in context: A327111 A326853 A326879 * A326876 A026486 A103838

Adjacent sequences: A326872 A326873 A326874 * A326876 A326877 A326878

KEYWORD

nonn

AUTHOR

Gus Wiseman, Jul 29 2019

STATUS

approved