OFFSET
1,3
COMMENTS
A set-system is a finite set of finite nonempty sets. Its elements are sometimes called edges. 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}}. This sequence gives all BII-numbers (defined below) of pairwise intersecting set-systems whose dual is strict and pairwise intersecting.
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 set-system 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.
EXAMPLE
The sequence of all set-systems that are pairwise intersecting, cointersecting, and costrict, together with their BII-numbers, begins:
0: {}
1: {{1}}
2: {{2}}
5: {{1},{1,2}}
6: {{2},{1,2}}
8: {{3}}
17: {{1},{1,3}}
24: {{3},{1,3}}
34: {{2},{2,3}}
40: {{3},{2,3}}
52: {{1,2},{1,3},{2,3}}
69: {{1},{1,2},{1,2,3}}
70: {{2},{1,2},{1,2,3}}
81: {{1},{1,3},{1,2,3}}
84: {{1,2},{1,3},{1,2,3}}
85: {{1},{1,2},{1,3},{1,2,3}}
88: {{3},{1,3},{1,2,3}}
98: {{2},{2,3},{1,2,3}}
100: {{1,2},{2,3},{1,2,3}}
102: {{2},{1,2},{2,3},{1,2,3}}
MATHEMATICA
dual[eds_]:=Table[First/@Position[eds, x], {x, Union@@eds}];
bpe[n_]:=Join@@Position[Reverse[IntegerDigits[n, 2]], 1];
stableQ[u_, Q_]:=!Apply[Or, Outer[#1=!=#2&&Q[#1, #2]&, u, u, 1], {0, 1}];
Select[Range[0, 10000], UnsameQ@@dual[bpe/@bpe[#]]&&stableQ[bpe/@bpe[#], Intersection[#1, #2]=={}&]&&stableQ[dual[bpe/@bpe[#]], Intersection[#1, #2]=={}&]&]
CROSSREFS
KEYWORD
nonn
AUTHOR
Gus Wiseman, Aug 18 2019
STATUS
approved