A327359 Triangle read by rows where T(n,k) is the number of unlabeled antichains of nonempty sets covering n vertices with vertex-connectivity exactly k.

1, 1, 0, 1, 1, 0, 2, 1, 2, 0, 6, 4, 4, 6, 0, 23, 29, 37, 37, 54, 0

Offset

0,7

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
   1  1  0
   2  1  2  0
   6  4  4  6  0
  23 29 37 37 54  0
Row n = 4 counts the following antichains:
{1}{234}      {14}{234}        {134}{234}           {1234}
{12}{34}      {13}{24}{34}     {13}{14}{234}        {12}{134}{234}
{1}{2}{34}    {14}{24}{34}     {12}{13}{24}{34}     {124}{134}{234}
{1}{24}{34}   {14}{23}{24}{34} {13}{14}{23}{24}{34} {12}{13}{14}{234}
{1}{2}{3}{4}                                        {123}{124}{134}{234}
{1}{23}{24}{34}                                     {12}{13}{14}{23}{24}{34}

Crossrefs

Row sums are A261005, or A006602 if empty edges are allowed.

Column k = 0 is A327426.

Column k = 1 is A327436.

Column k = n - 1 is A327425.

The labeled version is A327351.

Cf. A003465, A006126, A014466, A048143, A293993, A323818, A326704, A327125, A327334, A327336, A327350, A327358.

Sequence in context: A246272 A055135 A334873 * A197522 A121310 A356919

Adjacent sequences: A327356 A327357 A327358 * A327360 A327361 A327362

Keyword

nonn,tabl,more

Author

Gus Wiseman, Sep 10 2019

Status

approved