OFFSET
1,1
COMMENTS
Differs from A327109 in lacking 116, 117, 118, 119, 124, 125, 126, 127, ...
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 (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.
The spanning edge-connectivity of a set-system is the minimum number of edges that must be removed (without removing incident vertices) to obtain a disconnected or empty set-system.
EXAMPLE
The sequence of all set-systems with spanning edge-connectivity 2 together with their BII-numbers begins:
52: {{1,2},{1,3},{2,3}}
53: {{1},{1,2},{1,3},{2,3}}
54: {{2},{1,2},{1,3},{2,3}}
55: {{1},{2},{1,2},{1,3},{2,3}}
60: {{1,2},{3},{1,3},{2,3}}
61: {{1},{1,2},{3},{1,3},{2,3}}
62: {{2},{1,2},{3},{1,3},{2,3}}
63: {{1},{2},{1,2},{3},{1,3},{2,3}}
84: {{1,2},{1,3},{1,2,3}}
85: {{1},{1,2},{1,3},{1,2,3}}
86: {{2},{1,2},{1,3},{1,2,3}}
87: {{1},{2},{1,2},{1,3},{1,2,3}}
92: {{1,2},{3},{1,3},{1,2,3}}
93: {{1},{1,2},{3},{1,3},{1,2,3}}
94: {{2},{1,2},{3},{1,3},{1,2,3}}
95: {{1},{2},{1,2},{3},{1,3},{1,2,3}}
100: {{1,2},{2,3},{1,2,3}}
101: {{1},{1,2},{2,3},{1,2,3}}
102: {{2},{1,2},{2,3},{1,2,3}}
103: {{1},{2},{1,2},{2,3},{1,2,3}}
MATHEMATICA
bpe[n_]:=Join@@Position[Reverse[IntegerDigits[n, 2]], 1];
csm[s_]:=With[{c=Select[Tuples[Range[Length[s]], 2], And[OrderedQ[#], UnsameQ@@#, Length[Intersection@@s[[#]]]>0]&]}, If[c=={}, s, csm[Sort[Append[Delete[s, List/@c[[1]]], Union@@s[[c[[1]]]]]]]]];
spanEdgeConn[vts_, eds_]:=Length[eds]-Max@@Length/@Select[Subsets[eds], Union@@#!=vts||Length[csm[#]]!=1&];
Select[Range[0, 100], spanEdgeConn[Union@@bpe/@bpe[#], bpe/@bpe[#]]==2&]
CROSSREFS
Positions of 2's in A327144.
Graphs with spanning edge-connectivity >= 2 are counted by A095983.
Graphs with spanning edge-connectivity 2 are counted by A327146.
Set-systems with spanning edge-connectivity 2 are counted by A327130.
BII-numbers for non-spanning edge-connectivity 2 are A327097.
BII-numbers for spanning edge-connectivity >= 2 are A327109.
BII-numbers for spanning edge-connectivity 1 are A327111.
KEYWORD
nonn
AUTHOR
Gus Wiseman, Aug 23 2019
STATUS
approved