A327099 - OEIS

A327099

BII-numbers of set-systems with non-spanning edge-connectivity 1.

1, 2, 4, 7, 8, 16, 22, 23, 25, 28, 29, 30, 31, 32, 37, 39, 42, 44, 45, 46, 47, 49, 50, 51, 57, 58, 59, 64, 67, 73, 74, 75, 76, 77, 78, 79, 82, 83, 90, 91, 97, 99, 105, 107, 128, 256, 262, 263, 278, 279, 280, 281, 284, 285, 286, 287, 292, 293, 294, 295, 300

(list; graph; refs; listen; history; text; internal format)

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 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 non-spanning edge-connectivity of a set-system is the minimum number of edges that must be removed (along with any non-covered vertices) to result in a disconnected or empty set-system.

LINKS

Table of n, a(n) for n=1..61.

EXAMPLE

The sequence of all set-systems with non-spanning edge-connectivity 1 together with their BII-numbers begins:

1: {{1}}

2: {{2}}

4: {{1,2}}

7: {{1},{2},{1,2}}

8: {{3}}

16: {{1,3}}

22: {{2},{1,2},{1,3}}

23: {{1},{2},{1,2},{1,3}}

25: {{1},{3},{1,3}}

28: {{1,2},{3},{1,3}}

29: {{1},{1,2},{3},{1,3}}

30: {{2},{1,2},{3},{1,3}}

31: {{1},{2},{1,2},{3},{1,3}}

32: {{2,3}}

37: {{1},{1,2},{2,3}}

39: {{1},{2},{1,2},{2,3}}

42: {{2},{3},{2,3}}

44: {{1,2},{3},{2,3}}

45: {{1},{1,2},{3},{2,3}}

46: {{2},{1,2},{3},{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]]]]]]]]];

edgeConn[y_]:=If[Length[csm[bpe/@y]]!=1, 0, Length[y]-Max@@Length/@Select[Union[Subsets[y]], Length[csm[bpe/@#]]!=1&]];

Select[Range[0, 100], edgeConn[bpe[#]]==1&]

CROSSREFS

Positions of 1's in A326787.

Simple graphs with non-spanning edge-connectivity 1 are A327071.

BII-numbers for non-spanning edge-connectivity >= 1 are A326749.

BII-numbers for non-spanning edge-connectivity 2 are A327097.

BII-numbers for spanning edge-connectivity 1 are A327111.

BII-numbers for vertex-connectivity 1 are A327114.

Covering set-systems with non-spanning edge-connectivity 1 are counted by A327129.

Cf. A048793, A052446, A070939, A322338, A326031, A327108.

Sequence in context: A241548 A373801 A019539 * A338888 A369703 A337744

Adjacent sequences: A327096 A327097 A327098 * A327100 A327101 A327102

KEYWORD

nonn

AUTHOR

Gus Wiseman, Aug 21 2019

STATUS

approved