

A327080


BIInumbers of maximal uniform setsystems (or complete hypergraphs).


3



0, 1, 2, 3, 4, 8, 9, 10, 11, 16, 32, 52, 64, 128, 129, 130, 131, 136, 137, 138, 139, 256, 512, 772, 1024, 2048, 2320, 2592, 2868, 4096, 8192, 13376, 16384, 32768, 32769, 32770, 32771, 32776, 32777, 32778, 32779, 32896, 32897, 32898, 32899, 32904, 32905, 32906
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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 setsystem with BIInumber n to be obtained by taking the binary indices of each binary index of n. Every setsystem (finite set of finite nonempty sets) has a different BIInumber. 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 BIInumber of {{2},{1,3}} is 18. Elements of a setsystem are sometimes called edges.
A setsystem is uniform if all edges have the same size.


LINKS

Table of n, a(n) for n=1..48.


EXAMPLE

The sequence of all maximal uniform setsystems together with their BIInumbers begins:
0: {}
1: {{1}}
2: {{2}}
3: {{1},{2}}
4: {{1,2}}
8: {{3}}
9: {{1},{3}}
10: {{2},{3}}
11: {{1},{2},{3}}
16: {{1,3}}
32: {{2,3}}
52: {{1,2},{1,3},{2,3}}
64: {{1,2,3}}
128: {{4}}
129: {{1},{4}}
130: {{2},{4}}
131: {{1},{2},{4}}
136: {{3},{4}}
137: {{1},{3},{4}}
138: {{2},{3},{4}}


MATHEMATICA

bpe[n_]:=Join@@Position[Reverse[IntegerDigits[n, 2]], 1];
Select[Range[0, 100], With[{sys=bpe/@bpe[#]}, #==0SameQ@@Length/@sys&&Length[sys]==Binomial[Length[Union@@sys], Length[First[sys]]]]&]


CROSSREFS

BIInumbers of uniform setsystems are A326783.
The normal case (where the edges cover an initial interval) is A327081.
Cf. A000120, A048793, A070939, A326031, A326784, A326785, A327041.
Sequence in context: A004826 A326783 A326785 * A291621 A329268 A288860
Adjacent sequences: A327077 A327078 A327079 * A327081 A327082 A327083


KEYWORD

nonn


AUTHOR

Gus Wiseman, Aug 20 2019


STATUS

approved



