A327358 Triangle read by rows where T(n,k) is the number of unlabeled antichains of nonempty sets covering n vertices with vertex-connectivity >= k.

1, 1, 0, 2, 1, 0, 5, 3, 2, 0, 20, 14, 10, 6, 0, 180, 157, 128, 91, 54, 0

Offset

0,4

Comments

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.

Links

Table of n, a(n) for n=0..20.

Example

Triangle begins:
    1
    1   0
    2   1   0
    5   3   2   0
   20  14  10   6   0
  180 157 128  91  54   0
Non-isomorphic representatives of the antichains counted in row n = 4:
  {1234}          {1234}           {1234}           {1234}
  {1}{234}        {12}{134}        {123}{124}       {12}{134}{234}
  {12}{34}        {123}{124}       {12}{13}{234}    {123}{124}{134}
  {12}{134}       {12}{13}{14}     {12}{134}{234}   {12}{13}{14}{234}
  {123}{124}      {12}{13}{24}     {123}{124}{134}  {123}{124}{134}{234}
  {1}{2}{34}      {12}{13}{234}    {12}{13}{24}{34} {12}{13}{14}{23}{24}{34}
  {2}{13}{14}     {12}{134}{234}   {12}{13}{14}{234}
  {12}{13}{14}    {123}{124}{134}  {12}{13}{14}{23}{24}
  {12}{13}{24}    {12}{13}{14}{23} {123}{124}{134}{234}
  {1}{2}{3}{4}    {12}{13}{24}{34} {12}{13}{14}{23}{24}{34}
  {12}{13}{234}   {12}{13}{14}{234}
  {12}{134}{234}  {12}{13}{14}{23}{24}
  {123}{124}{134} {123}{124}{134}{234}
  {4}{12}{13}{23} {12}{13}{14}{23}{24}{34}
  {12}{13}{14}{23}
  {12}{13}{24}{34}
  {12}{13}{14}{234}
  {12}{13}{14}{23}{24}
  {123}{124}{134}{234}
  {12}{13}{14}{23}{24}{34}

Crossrefs

Column k = 0 is A261005, or A006602 if empty edges are allowed.

Column k = 1 is A261006 (clutters), if we assume A261006(0) = A261006(1) = 0.

Column k = 2 is A305028 (blobs), if we assume A305028(0) = A305028(2) = 0.

Column k = n - 1 is A327425 (cointersecting).

The labeled version is A327350.

Negated first differences of rows are A327359.

Cf. A006126, A055621, A120338, A293606, A293993, A327334, A327351, A327356.

Sequence in context: A073583 A324162 A060136 * A256664 A226783 A245972

Adjacent sequences: A327355 A327356 A327357 * A327359 A327360 A327361

Keyword

nonn,tabl,more

Author

Gus Wiseman, Sep 09 2019

Status

approved