

A326872


BIInumbers of connectedness systems.


14



0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 24, 25, 26, 27, 32, 33, 34, 35, 40, 41, 42, 43, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99
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OFFSET

1,3


COMMENTS

We define a connectedness system (investigated by Vim van Dam in 2002) to be a set of finite nonempty sets (edges) that is closed under taking the union of any two overlapping edges.
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 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.
The enumeration of these setsystems by number of covered vertices is given by A326870.


LINKS



EXAMPLE

The sequence of all connectedness systems together with their BIInumbers begins:
0: {}
1: {{1}}
2: {{2}}
3: {{1},{2}}
4: {{1,2}}
5: {{1},{1,2}}
6: {{2},{1,2}}
7: {{1},{2},{1,2}}
8: {{3}}
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}}
16: {{1,3}}
17: {{1},{1,3}}
18: {{2},{1,3}}
19: {{1},{2},{1,3}}
24: {{3},{1,3}}
25: {{1},{3},{1,3}}
26: {{2},{3},{1,3}}
27: {{1},{2},{3},{1,3}}
32: {{2,3}}


MATHEMATICA

bpe[n_]:=Join@@Position[Reverse[IntegerDigits[n, 2]], 1];
connsysQ[eds_]:=SubsetQ[eds, Union@@@Select[Tuples[eds, 2], Intersection@@#!={}&]];
Select[Range[0, 100], connsysQ[bpe/@bpe[#]]&]


CROSSREFS

Connectedness systems are counted by A326866, with unlabeled version A326867.
The case without singletons is A326873.
Setsystems closed under union are counted by A102896, with BII numbers A326875.


KEYWORD

nonn


AUTHOR



STATUS

approved



