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A326875
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BII-numbers of set-systems that are closed under union.
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14
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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
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1,3
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COMMENTS
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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.
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LINKS
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EXAMPLE
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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}}
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MATHEMATICA
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bpe[n_]:=Join@@Position[Reverse[IntegerDigits[n, 2]], 1];
Select[Range[0, 100], SubsetQ[bpe/@bpe[#], Union@@@Tuples[bpe/@bpe[#], 2]]&]
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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