A275307 -id:A275307 - cp's OEIS frontend

A327113 Number of set-systems covering n vertices with cut-connectivity 2.

0, 0, 4, 0, 4752

Offset: 0

Gus Wiseman, Aug 24 2019

A set-system is a finite set of finite nonempty sets. Elements of a set-system are sometimes called edges. The cut-connectivity of a set-system is the minimum number of vertices that must be removed (along with any empty or duplicate edges) to obtain a disconnected or empty set-system. Except for cointersecting set-systems (A327040), this is the same as vertex-connectivity (A327334, A327051).

			The a(2) = 4 set-systems:
  {{1,2}}
  {{1},{1,2}}
  {{2},{1,2}}
  {{1},{2},{1,2}}

Covering graphs with cut-connectivity >= 2 are A013922, if we assume A013922(2) = 1.
Covering antichains (blobs) with cut-connectivity >= 2 are A275307, if we assume A275307(1) = 0.
2-vertex-connected integer partitions are A322387.
Connected covering set-systems are A323818.
Covering set-systems with cut-connectivity >= 2 are A327112.
The cut-connectivity of the set-system with BII-number n is A326786(n).
BII-numbers of set-systems with cut-connectivity 2 are A327082.
Cf. A002218, A003465, A048143, A259862, A322389, A327101, A327113, A327126, A327128, A327130.

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]]]]]]]]];
vConn[sys_]:=If[Length[csm[sys]]!=1,0,Min@@Length/@Select[Subsets[Union@@sys],Function[del,Length[csm[DeleteCases[DeleteCases[sys,Alternatives@@del,{2}],{}]]]!=1]]];
Table[Length[Select[Subsets[Subsets[Range[n],{1,n}]],Union@@#==Range[n]&&vConn[#]==2&]],{n,0,3}]

A318697 Number of ways to partition a hypertree spanning n vertices into hypertrees.

Original entry on oeis.org

1, 1, 7, 93, 1856, 49753, 1679441, 68463769, 3273695758, 179710285011, 11141016392749, 769939840667473, 58695964339179805, 4893452980658819151, 442915168219228586581, 43255083632741702266097, 4533695508041747494704359, 507638249638364368312476913

Offset: 1

Gus Wiseman, Aug 31 2018

nonn

			The a(3) = 7 hypertree partitions:
  {{{1,2,3}}}
  {{{1,2},{1,3}}}
  {{{1,2},{2,3}}}
  {{{1,3},{2,3}}}
  {{{1,2}},{{1,3}}}
  {{{1,2}},{{2,3}}}
  {{{1,3}},{{2,3}}}

Cf. A000272, A030019, A048143, A134954, A275307, A317631, A317632, A317634, A317635, A317677.

trct[n_]:=Sum[StirlingS2[n-1,i]*n^(i-1),{i,0,n-1}];
numSetPtnsOfType[ptn_]:=Total[ptn]!/Times@@Factorial/@ptn/Times@@Factorial/@Length/@Split[ptn];
Table[Sum[n^(Length[ptn]-1)*Product[trct[s+1],{s,ptn}]*numSetPtnsOfType[ptn],{ptn,IntegerPartitions[n-1]}],{n,20}]

A327350 Triangle read by rows where T(n,k) is the number of antichains of nonempty sets covering n vertices with vertex-connectivity >= k.

Original entry on oeis.org

1, 1, 0, 2, 1, 0, 9, 5, 2, 0, 114, 84, 44, 17, 0, 6894, 6348, 4983, 3141, 1451, 0, 7785062

Offset: 0

Gus Wiseman, Sep 09 2019

An antichain is a set of sets, none of which is a subset of any other. It is covering if there are no isolated vertices.
The vertex-connectivity of a set-system is the minimum number of vertices that must be removed (along with any empty or duplicate edges) to obtain a non-connected set-system or singleton. Note that this means a single node has vertex-connectivity 0.
If empty edges are allowed, we have T(0,0) = 2.

			Triangle begins:
     1
     1    0
     2    1    0
     9    5    2    0
   114   84   44   17    0
  6894 6348 4983 3141 1451    0
The antichains counted in row n = 3:
  {123}         {123}         {123}
  {1}{23}       {12}{13}      {12}{13}{23}
  {2}{13}       {12}{23}
  {3}{12}       {13}{23}
  {12}{13}      {12}{13}{23}
  {12}{23}
  {13}{23}
  {1}{2}{3}
  {12}{13}{23}

Column k = 0 is A307249, or A006126 if empty edges are allowed.
Column k = 1 is A048143 (clutters), if we assume A048143(0) = A048143(1) = 0.
Column k = 2 is A275307 (blobs), if we assume A275307(1) = A275307(2) = 0.
Column k = n - 1 is A327020 (cointersecting antichains).
The unlabeled version is A327358.
Negated first differences of rows are A327351.
BII-numbers of antichains are A326704.
Antichain covers are A006126.
Cf. A003465, A014466, A120338, A293606, A293993, A319639, A323818, A327112, A327125, A327334, A327336, A327352, A327356, A327357, A327358.

csm[s_]:=With[{c=Select[Subsets[Range[Length[s]],{2}],Length[Intersection@@s[[#]]]>0&]},If[c=={},s,csm[Sort[Append[Delete[s,List/@c[[1]]],Union@@s[[c[[1]]]]]]]]];
stableSets[u_,Q_]:=If[Length[u]==0,{{}},With[{w=First[u]},Join[stableSets[DeleteCases[u,w],Q],Prepend[#,w]&/@stableSets[DeleteCases[u,r_/;r==w||Q[r,w]||Q[w,r]],Q]]]];
vertConnSys[vts_,eds_]:=Min@@Length/@Select[Subsets[vts],Function[del,Length[del]==Length[vts]-1||csm[DeleteCases[DeleteCases[eds,Alternatives@@del,{2}],{}]]!={Complement[vts,del]}]];
Table[Length[Select[stableSets[Subsets[Range[n],{1,n}],SubsetQ],Union@@#==Range[n]&&vertConnSys[Range[n],#]>=k&]],{n,0,4},{k,0,n}]

a(21) from Robert Price, May 24 2021

A322117 Number of non-isomorphic blobs (2-connected weak antichains) of multisets of weight n.

Original entry on oeis.org

1, 1, 3, 4, 8, 8, 21, 27, 79, 185, 554

Offset: 0

Gus Wiseman, Nov 26 2018

			Non-isomorphic representatives of the a(1) = 1 through a(6) = 21 blobs:
  (1)  (11)    (111)      (1111)        (11111)          (111111)
       (12)    (122)      (1122)        (11222)          (111222)
       (1)(1)  (123)      (1222)        (12222)          (112222)
               (1)(1)(1)  (1233)        (12233)          (112233)
                          (1234)        (12333)          (122222)
                          (11)(11)      (12344)          (122333)
                          (12)(12)      (12345)          (123333)
                          (1)(1)(1)(1)  (1)(1)(1)(1)(1)  (123344)
                                                         (123444)
                                                         (123455)
                                                         (123456)
                                                         (111)(111)
                                                         (112)(122)
                                                         (122)(122)
                                                         (123)(123)
                                                         (123)(233)
                                                         (134)(234)
                                                         (11)(11)(11)
                                                         (12)(12)(12)
                                                         (12)(13)(23)
                                                         (1)(1)(1)(1)(1)(1)

Gus Wiseman, Every Clutter Is a Tree of Blobs, The Mathematica Journal, Vol. 19, 2017.

Cf. A002218, A007718, A013922, A275307, A286520, A293994, A304118, A304887, A319719, A319721, A322110, A322118.

A322388 Heinz numbers of 2-vertex-connected integer partitions.

Original entry on oeis.org

13, 29, 37, 39, 43, 47, 61, 65, 71, 73, 79, 87, 89, 91, 101, 107, 111, 113, 117, 129, 137, 139, 149, 151, 163, 167, 169, 173, 181, 183, 185, 193, 195, 197, 199, 203, 213, 223, 229, 233, 235, 237, 239, 247, 251, 257, 259, 261, 263, 267, 269, 271, 273, 281

Offset: 1

Gus Wiseman, Dec 05 2018

nonn

The Heinz number of an integer partition (y_1, ..., y_k) is prime(y_1) * ... * prime(y_k).

An integer partition is 2-vertex-connected if the prime factorizations of the parts form a connected hypergraph that is still connected if any single prime number is divided out of all the parts (and any parts then equal to 1 are removed).

			The sequence of all 2-vertex-connected integer partitions begins: (1), (6), (10), (12), (6,2), (14), (15), (18), (6,3), (20), (21), (22), (10,2), (24), (6,4), (26), (28), (12,2), (30), (6,2,2), (14,2), (33), (34), (35), (36), (38), (39), (6,6), (40), (42), (18,2), (12,3), (44), (6,3,2), (45), (46).

Wikipedia, k-vertex-connected graph

Cf. A003963, A013922, A056239, A095983, A112798, A218970, A275307, A304716, A305078, A305079, A322336, A322338, A322387, A322389, A322390.

primeMS[n_]:=If[n==1,{},Flatten[Cases[FactorInteger[n],{p_,k_}:>Table[PrimePi[p],{k}]]]];
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]]]]]]]]];
vertConn[y_]:=If[Length[csm[primeMS/@y]]!=1,0,Min@@Length/@Select[Subsets[Union@@primeMS/@y],Function[del,Length[csm[DeleteCases[DeleteCases[primeMS/@y,Alternatives@@del,{2}],{}]]]!=1]]]
Select[Range[100],vertConn[primeMS[#]]>1&]

A010357 Number of unlabeled nonseparable (or 2-connected) loopless multigraphs with n edges.

Original entry on oeis.org

1, 1, 2, 3, 6, 14, 32, 90, 279, 942, 3468, 13777, 57747, 254671, 1170565, 5580706, 27487418, 139477796, 727458338, 3893078684, 21346838204, 119787629215, 687200870250

Offset: 1

N. J. A. Sloane

Original name: Multi-edge stars with n edges.

			From _Andrew Howroyd_, Nov 23 2020: (Start)
The a(1) = 1 graph is a single edge (K_2 = P_2).
The a(2) = 1 graph is a double edge.
The a(3) = 2 graphs are a triple edge and the triangle (K_3).
The a(4) = 3 graphs are a quadruple edge, a triangle with one double edge and the square (C_4).
(End)

George A. Baker Jr. and John M. Kincaid, The continuous-spin Ising model, g0:phi4:d field theory, and the renormalization group, J. Statist. Phys. 24 (1981), no. 3, 469-528.
Brendan McKay and Adolfo Piperno, nauty and Traces, programs for computing automorphism groups of graphs and digraphs.
Gus Wiseman, Non-isomorphic representatives of the a(1) = 1 through a(6) = 14 unlabeled 2-connected multigraphs.

Row sums of A339160.
Cf. A050535, A076864, A010355, A010359.
A002218 counts unlabeled 2-connected graphs.
A013922 counts labeled 2-connected graphs.
A322140 is a labeled version.
Cf. A002905, A006444, A007718, A275307, A304887.

Name changed by Andrew Howroyd, Dec 05 2020
a(11)-a(20) added using geng/multig from nauty by Andrew Howroyd, Dec 05 2020
a(21)-a(23) from Sean A. Irvine, Apr 18 2024

A317677 Fixed point of a shifted hypertree transform.

Original entry on oeis.org

1, 1, 4, 32, 402, 7038, 160114, 4522578, 153640590, 6132546770, 282517271694, 14812447505646, 873934551644074, 57486823088667270, 4183353479821220130, 334572221351085006242, 29242220614539638127294, 2779426070382982579163202, 286058737295150226682469518

Offset: 1

Gus Wiseman, Aug 04 2018

The hypertree transform H(a) of a sequence a is given by H(a)(n) = Sum_p n^(k-1) Prod_i a(|p_i|+1), where the sum is over all set partitions U(p_1, ..., p_k) = {1, ..., n-1}.

Alois P. Heinz, Table of n, a(n) for n = 1..305

Cf. A000272, A030019, A048143, A134954, A275307, A293510, A317631, A317632, A317634, A317635, A317671.

b:= proc(n, k) option remember; `if`(n=0, 1/k, add(
      a(j)*b(n-j, k)*binomial(n-1, j-1)*k, j=1..n))
    end:
a:= n-> b(n-1, n):
seq(a(n), n=1..20);  # Alois P. Heinz, Aug 21 2019

numSetPtnsOfType[ptn_]:=Total[ptn]!/Times@@Factorial/@ptn/Times@@Factorial/@Length/@Split[ptn];
a[n_]:=a[n]=Sum[n^(Length[ptn]-1)*numSetPtnsOfType[ptn]*Product[a[s],{s,ptn}],{ptn,IntegerPartitions[n-1]}];
Array[a,20]
(* Second program: *)
b[n_, k_] := b[n, k] = If[n == 0, 1/k, Sum[
     a[j]*b[n - j, k]*Binomial[n - 1, j - 1]*k, {j, 1, n}]];
a[n_] := b[n - 1, n];
Array[a, 20] (* Jean-François Alcover, May 10 2021, after Alois P. Heinz *)

A322110 Number of non-isomorphic connected multiset partitions of weight n that cannot be capped by a tree.

Original entry on oeis.org

1, 1, 3, 6, 15, 32, 86, 216, 628, 1836, 5822

Offset: 0

Gus Wiseman, Nov 26 2018

The weight of a multiset partition is the sum of sizes of its parts. Weight is generally not the same as number of vertices.

The density of a multiset partition is defined to be the sum of numbers of distinct elements in each part, minus the number of parts, minus the total number of distinct elements in the whole partition. A multiset partition is a tree if it has more than one part, is connected, and has density -1. A cap is a certain kind of non-transitive coarsening of a multiset partition. For example, the four caps of {{1,1},{1,2},{2,2}} are {{1,1},{1,2},{2,2}}, {{1,1},{1,2,2}}, {{1,1,2},{2,2}}, {{1,1,2,2}}. - Gus Wiseman, Feb 05 2021

			The multiset partition C = {{1,1},{1,2,3},{2,3,3}} is not a tree but has the cap {{1,1},{1,2,3,3}} which is a tree, so C is not counted under a(8).
Non-isomorphic representatives of the a(1) = 1 through a(5) = 32 multiset partitions:
  {{1}}  {{1,1}}    {{1,1,1}}      {{1,1,1,1}}        {{1,1,1,1,1}}
         {{1,2}}    {{1,2,2}}      {{1,1,2,2}}        {{1,1,2,2,2}}
         {{1},{1}}  {{1,2,3}}      {{1,2,2,2}}        {{1,2,2,2,2}}
                    {{1},{1,1}}    {{1,2,3,3}}        {{1,2,2,3,3}}
                    {{2},{1,2}}    {{1,2,3,4}}        {{1,2,3,3,3}}
                    {{1},{1},{1}}  {{1},{1,1,1}}      {{1,2,3,4,4}}
                                   {{1,1},{1,1}}      {{1,2,3,4,5}}
                                   {{1},{1,2,2}}      {{1},{1,1,1,1}}
                                   {{1,2},{1,2}}      {{1,1},{1,1,1}}
                                   {{2},{1,2,2}}      {{1},{1,2,2,2}}
                                   {{3},{1,2,3}}      {{1,2},{1,2,2}}
                                   {{1},{1},{1,1}}    {{2},{1,1,2,2}}
                                   {{1},{2},{1,2}}    {{2},{1,2,2,2}}
                                   {{2},{2},{1,2}}    {{2},{1,2,3,3}}
                                   {{1},{1},{1},{1}}  {{2,2},{1,2,2}}
                                                      {{2,3},{1,2,3}}
                                                      {{3},{1,2,3,3}}
                                                      {{4},{1,2,3,4}}
                                                      {{1},{1},{1,1,1}}
                                                      {{1},{1,1},{1,1}}
                                                      {{1},{1},{1,2,2}}
                                                      {{1},{2},{1,2,2}}
                                                      {{2},{1,2},{1,2}}
                                                      {{2},{1,2},{2,2}}
                                                      {{2},{2},{1,2,2}}
                                                      {{2},{3},{1,2,3}}
                                                      {{3},{1,3},{2,3}}
                                                      {{3},{3},{1,2,3}}
                                                      {{1},{1},{1},{1,1}}
                                                      {{1},{2},{2},{1,2}}
                                                      {{2},{2},{2},{1,2}}
                                                      {{1},{1},{1},{1},{1}}

Gus Wiseman, Every Clutter Is a Tree of Blobs, The Mathematica Journal, Vol. 19, 2017.

Non-isomorphic tree multiset partitions are counted by A321229.
The weak-antichain case is counted by A322117.
The case without singletons is counted by A322118.
Cf. A002218, A007718, A013922, A030019, A056156, A275307, A304118, A304382, A304887, A305052, A305079, A321194.

Corrected by Gus Wiseman, Jan 27 2021

A322118 Number of non-isomorphic connected multiset partitions of weight n with no singletons that cannot be capped by a tree.

Original entry on oeis.org

1, 1, 2, 3, 7, 11, 29, 55, 155, 386, 1171

Offset: 0

Gus Wiseman, Nov 26 2018

The density of a multiset partition is defined to be the sum of numbers of distinct elements in each part, minus the number of parts, minus the total number of distinct elements in the whole partition. A multiset partition is a tree if it has more than one part, is connected, and has density -1. A cap is a certain kind of non-transitive coarsening of a multiset partition. For example, the four caps of {{1,1},{1,2},{2,2}} are {{1,1},{1,2},{2,2}}, {{1,1},{1,2,2}}, {{1,1,2},{2,2}}, {{1,1,2,2}}. - Gus Wiseman, Feb 05 2021

			The multiset partition C = {{1,1},{1,2,3},{2,3,3}} is not a tree but has the cap {{1,1},{1,2,3,3}} which is a tree, so C is not counted under a(8).
Non-isomorphic representatives of the a(2) = 2 through a(6) = 29 multiset partitions:
  {{1,1}}  {{1,1,1}}  {{1,1,1,1}}    {{1,1,1,1,1}}    {{1,1,1,1,1,1}}
  {{1,2}}  {{1,2,2}}  {{1,1,2,2}}    {{1,1,2,2,2}}    {{1,1,1,2,2,2}}
           {{1,2,3}}  {{1,2,2,2}}    {{1,2,2,2,2}}    {{1,1,2,2,2,2}}
                      {{1,2,3,3}}    {{1,2,2,3,3}}    {{1,1,2,2,3,3}}
                      {{1,2,3,4}}    {{1,2,3,3,3}}    {{1,2,2,2,2,2}}
                      {{1,1},{1,1}}  {{1,2,3,4,4}}    {{1,2,2,3,3,3}}
                      {{1,2},{1,2}}  {{1,2,3,4,5}}    {{1,2,3,3,3,3}}
                                     {{1,1},{1,1,1}}  {{1,2,3,3,4,4}}
                                     {{1,2},{1,2,2}}  {{1,2,3,4,4,4}}
                                     {{2,2},{1,2,2}}  {{1,2,3,4,5,5}}
                                     {{2,3},{1,2,3}}  {{1,2,3,4,5,6}}
                                                      {{1,1},{1,1,1,1}}
                                                      {{1,1,1},{1,1,1}}
                                                      {{1,1,2},{1,2,2}}
                                                      {{1,2},{1,1,2,2}}
                                                      {{1,2},{1,2,2,2}}
                                                      {{1,2},{1,2,3,3}}
                                                      {{1,2,2},{1,2,2}}
                                                      {{1,2,3},{1,2,3}}
                                                      {{1,2,3},{2,3,3}}
                                                      {{1,3,4},{2,3,4}}
                                                      {{2,2},{1,1,2,2}}
                                                      {{2,2},{1,2,2,2}}
                                                      {{2,3},{1,2,3,3}}
                                                      {{3,3},{1,2,3,3}}
                                                      {{3,4},{1,2,3,4}}
                                                      {{1,1},{1,1},{1,1}}
                                                      {{1,2},{1,2},{1,2}}
                                                      {{1,2},{1,3},{2,3}}

Gus Wiseman, Every Clutter Is a Tree of Blobs, The Mathematica Journal, Vol. 19, 2017.

Non-isomorphic tree multiset partitions are counted by A321229, or A321231 without singletons.
The version with singletons is A322110.
The weak-antichain case is counted by A322138, or A322117 with singletons.
Cf. A002218, A007718, A013922, A030019, A275307, A293994, A304118, A304382, A304887, A305079, A319719, A319721, A321194.

Definition corrected by Gus Wiseman, Feb 05 2021

A322391 Number of integer partitions of n with edge-connectivity 1.

Original entry on oeis.org

1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 1, 3, 2, 3, 3, 9, 3, 14, 8, 17, 13, 35, 17, 49, 35, 67, 53, 114, 69

Offset: 1

Gus Wiseman, Dec 05 2018

The edge-connectivity of an integer partition is the minimum number of parts that must be removed so that the prime factorizations of the remaining parts form a disconnected (or empty) hypergraph.

			The a(20) = 8 integer partitions:
  (20),
  (12,3,3,2), (9,6,3,2), (8,6,3,3),
  (6,4,4,3,3),
  (6,4,3,3,2,2), (6,3,3,3,3,2),
  (6,3,3,2,2,2,2).

Wikipedia, k-edge-connected graph

Cf. A007718, A013922, A054921, A095983, A218970, A275307, A304716, A305078, A305079, A322335, A322336, A322387, A322390.

primeMS[n_]:=If[n==1,{},Flatten[Cases[FactorInteger[n],{p_,k_}:>Table[PrimePi[p],{k}]]]];
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[primeMS/@y]]!=1,0,Length[y]-Max@@Length/@Select[Union[Subsets[y]],Length[csm[primeMS/@#]]!=1&]];
Table[Length[Select[IntegerPartitions[n],edgeConn[#]==1&]],{n,20}]

cp's OEIS Frontend

A327113 Number of set-systems covering n vertices with cut-connectivity 2.

Views

Author

Keywords

Comments

Examples

Crossrefs

Programs

Mathematica

A318697 Number of ways to partition a hypertree spanning n vertices into hypertrees.

Views

Author

Keywords

Examples

Crossrefs

Programs

Mathematica

A327350 Triangle read by rows where T(n,k) is the number of antichains of nonempty sets covering n vertices with vertex-connectivity >= k.

Views

Author

Keywords

Comments

Examples

Crossrefs

Programs

Mathematica

Extensions

A322117 Number of non-isomorphic blobs (2-connected weak antichains) of multisets of weight n.

Views

Author

Keywords

Examples

Links

Crossrefs

A322388 Heinz numbers of 2-vertex-connected integer partitions.

Views

Author

Keywords

Comments

Examples

Links

Crossrefs

Programs

Mathematica

A010357 Number of unlabeled nonseparable (or 2-connected) loopless multigraphs with n edges.

Views

Author

Keywords

Comments

Examples

Links

Crossrefs

Extensions

A317677 Fixed point of a shifted hypertree transform.

Views

Author

Keywords

Comments

Links

Crossrefs

Programs

Maple

Mathematica

A322110 Number of non-isomorphic connected multiset partitions of weight n that cannot be capped by a tree.

Views

Author

Keywords

Comments

Examples

Links

Crossrefs

Extensions

A322118 Number of non-isomorphic connected multiset partitions of weight n with no singletons that cannot be capped by a tree.

Views

Author

Keywords

Comments

Examples

Links