login
BII-numbers of finite T_0 topologies without their empty set.
2

%I #6 Aug 15 2019 07:30:11

%S 0,1,2,5,6,7,8,17,24,25,34,40,42,69,70,71,81,85,87,88,89,93,98,102,

%T 103,104,106,110,120,121,122,127,128,257,384,385,514,640,642,1029,

%U 1030,1031,1281,1285,1287,1408,1409,1413,1538,1542,1543,1664,1666,1670,1920

%N BII-numbers of finite T_0 topologies without their empty set.

%C A set-system is a finite set of finite nonempty sets. The dual of a set-system has, for each vertex, one edge consisting of the indices (or positions) of the edges containing that vertex. For example, the dual of {{1,2},{2,3}} is {{1},{1,2},{2}}. The T_0 condition means that the dual is strict (no repeated edges).

%C 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.

%H Gus Wiseman, <a href="/A327016/a327016.png">The first 100 finite T_0 topologies, represented as posets.</a>

%e The sequence of all finite T_0 topologies without their empty set together with their BII-numbers begins:

%e 0: {}

%e 1: {{1}}

%e 2: {{2}}

%e 5: {{1},{1,2}}

%e 6: {{2},{1,2}}

%e 7: {{1},{2},{1,2}}

%e 8: {{3}}

%e 17: {{1},{1,3}}

%e 24: {{3},{1,3}}

%e 25: {{1},{3},{1,3}}

%e 34: {{2},{2,3}}

%e 40: {{3},{2,3}}

%e 42: {{2},{3},{2,3}}

%e 69: {{1},{1,2},{1,2,3}}

%e 70: {{2},{1,2},{1,2,3}}

%e 71: {{1},{2},{1,2},{1,2,3}}

%e 81: {{1},{1,3},{1,2,3}}

%e 85: {{1},{1,2},{1,3},{1,2,3}}

%e 87: {{1},{2},{1,2},{1,3},{1,2,3}}

%e 88: {{3},{1,3},{1,2,3}}

%t bpe[n_]:=Join@@Position[Reverse[IntegerDigits[n,2]],1];

%t Select[Range[0,1000],UnsameQ@@dual[bpe/@bpe[#]]&&SubsetQ[bpe/@bpe[#],Union[Union@@@Tuples[bpe/@bpe[#],2],DeleteCases[Intersection@@@Tuples[bpe/@bpe[#],2],{}]]]&]

%Y T_0 topologies are A001035, with unlabeled version A000112.

%Y BII-numbers of topologies without their empty set are A326876.

%Y BII-numbers of T_0 set-systems are A326947.

%Y Cf. A001930, A048793, A306445, A316978, A319564, A326031, A326872, A326875, A326939, A326941, A326959.

%K nonn

%O 1,3

%A _Gus Wiseman_, Aug 14 2019