

A326788


BIInumbers of simple labeled graphs.


8



0, 4, 16, 20, 32, 36, 48, 52, 256, 260, 272, 276, 288, 292, 304, 308, 512, 516, 528, 532, 544, 548, 560, 564, 768, 772, 784, 788, 800, 804, 816, 820, 2048, 2052, 2064, 2068, 2080, 2084, 2096, 2100, 2304, 2308, 2320, 2324, 2336, 2340, 2352, 2356, 2560, 2564
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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 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.
Also numbers whose binary indices all belong to A018900.


LINKS

Table of n, a(n) for n=1..50.


EXAMPLE

The sequence of all simple labeled graphs together with their BIInumbers begins:
0: {}
4: {{1,2}}
16: {{1,3}}
20: {{1,2},{1,3}}
32: {{2,3}}
36: {{1,2},{2,3}}
48: {{1,3},{2,3}}
52: {{1,2},{1,3},{2,3}}
256: {{1,4}}
260: {{1,2},{1,4}}
272: {{1,3},{1,4}}
276: {{1,2},{1,3},{1,4}}
288: {{2,3},{1,4}}
292: {{1,2},{2,3},{1,4}}
304: {{1,3},{2,3},{1,4}}
308: {{1,2},{1,3},{2,3},{1,4}}
512: {{2,4}}
516: {{1,2},{2,4}}
528: {{1,3},{2,4}}
532: {{1,2},{1,3},{2,4}}


MATHEMATICA

bpe[n_]:=Join@@Position[Reverse[IntegerDigits[n, 2]], 1];
Select[Range[0, 100], SameQ[2, ##]&@@Length/@bpe/@bpe[#]&]


CROSSREFS

Cf. A000120, A006125, A006129, A018900, A048793, A062880, A070939, A309356 (same for MMnumbers), A322551, A326031, A326702.
Other BIInumbers: A309314 (hyperforests), A326701 (set partitions), A326703 (chains), A326704 (antichains), A326749 (connected), A326750 (clutters), A326751 (blobs), A326752 (hypertrees), A326754 (covers).
Sequence in context: A071966 A349521 A326781 * A039943 A193996 A232400
Adjacent sequences: A326785 A326786 A326787 * A326789 A326790 A326791


KEYWORD

nonn


AUTHOR

Gus Wiseman, Jul 25 2019


STATUS

approved



