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A326911
BII-numbers of set-systems with empty intersection.
3
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
1,2
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.
EXAMPLE
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}}
MATHEMATICA
bpe[n_]:=Join@@Position[Reverse[IntegerDigits[n, 2]], 1];
Select[Range[0, 100], #==0||Intersection@@bpe/@bpe[#]=={}&]
KEYWORD
nonn
AUTHOR
Gus Wiseman, Aug 04 2019
STATUS
approved