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A324173 Regular triangle read by rows where T(n,k) is the number of set partitions of {1,...,n} with k topologically connected components. 22
1, 0, 1, 0, 1, 1, 0, 1, 3, 1, 0, 2, 6, 6, 1, 0, 6, 15, 20, 10, 1, 0, 21, 51, 65, 50, 15, 1, 0, 85, 203, 252, 210, 105, 21, 1, 0, 385, 912, 1120, 938, 560, 196, 28, 1, 0, 1907, 4527, 5520, 4620, 2898, 1302, 336, 36, 1, 0, 10205, 24370, 29700, 24780, 15792, 7812, 2730, 540, 45, 1 (list; table; graph; refs; listen; history; text; internal format)
OFFSET

0,9

COMMENTS

A set partition is crossing if it contains a pair of blocks of the form {{...x...y...}, {...z...t...}} where x < z < y < t or z < x < t < y.

The topologically connected components of a set partition correspond to the blocks of its minimal non-crossing coarsening.

LINKS

Table of n, a(n) for n=0..65.

FindStat, The number of topologically connected components of a set partition.

EXAMPLE

Triangle begins:

     1

     0     1

     0     1     1

     0     1     3     1

     0     2     6     6     1

     0     6    15    20    10     1

     0    21    51    65    50    15     1

     0    85   203   252   210   105    21     1

     0   385   912  1120   938   560   196    28     1

     0  1907  4527  5520  4620  2898  1302   336    36     1

     0 10205 24370 29700 24780 15792  7812  2730   540    45     1

Row n = 4 counts the following set partitions:

  {{1234}}    {{1}{234}}  {{1}{2}{34}}  {{1}{2}{3}{4}}

  {{13}{24}}  {{12}{34}}  {{1}{23}{4}}

              {{123}{4}}  {{12}{3}{4}}

              {{124}{3}}  {{1}{24}{3}}

              {{134}{2}}  {{13}{2}{4}}

              {{14}{23}}  {{14}{2}{3}}

MATHEMATICA

croXQ[stn_]:=MatchQ[stn, {___, {___, x_, ___, y_, ___}, ___, {___, z_, ___, t_, ___}, ___}/; x<z<y<t||z<x<t<y];

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]]]]]]]]];

crosscmpts[stn_]:=csm[Union[Subsets[stn, {1}], Select[Subsets[stn, {2}], croXQ]]];

sps[{}]:={{}}; sps[set:{i_, ___}]:=Join@@Function[s, Prepend[#, s]&/@sps[Complement[set, s]]]/@Cases[Subsets[set], {i, ___}];

Table[Length[Select[sps[Range[n]], Length[crosscmpts[#]]==k&]], {n, 0, 8}, {k, 0, n}]

CROSSREFS

Row sums are A000110. Column k = 1 is A099947.

Cf. A000108, A001263, A002061, A002662, A007297, A016098, A048993, A054726, A293510, A305078, A305079, A323818.

Cf. A324166, A324167, A324169, A324170, A324171, A324172.

Sequence in context: A198345 A104416 A194582 * A293134 A293053 A144108

Adjacent sequences:  A324170 A324171 A324172 * A324174 A324175 A324176

KEYWORD

nonn,tabl

AUTHOR

Gus Wiseman, Feb 17 2019

STATUS

approved

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Last modified April 10 06:54 EDT 2021. Contains 342843 sequences. (Running on oeis4.)