A386634 Number of inseparable type set partitions of {1..n}.

0, 0, 1, 1, 5, 6, 37, 50, 345, 502, 3851, 5897, 49854, 79249, 730745, 1195147, 11915997, 19929390, 213332101, 363275555, 4150104224, 7172334477, 87003759195, 152231458128, 1952292972199, 3451893361661, 46625594567852, 83183249675125, 1179506183956655, 2120758970878892

Offset: 0

Author

Gus Wiseman, Aug 09 2025

Keywords

• nonn

Comments

A set partition is of inseparable type iff the underlying set has no permutation whose adjacent elements always belong to different blocks. Note that this only depends on the sizes of the blocks.

A set partition is also of inseparable type iff its greatest block size is at least 2 more than the sum of its other block sizes.

This is different from inseparable partitions (A325535) and partitions of inseparable type (A386638 or A025065).

Examples

			The a(2) = 1 through a(5) = 6 set partitions:
  {{1,2}}  {{1,2,3}}  {{1,2,3,4}}    {{1,2,3,4,5}}
                      {{1},{2,3,4}}  {{1},{2,3,4,5}}
                      {{1,2,3},{4}}  {{1,2,3,4},{5}}
                      {{1,2,4},{3}}  {{1,2,3,5},{4}}
                      {{1,3,4},{2}}  {{1,2,4,5},{3}}
                                     {{1,3,4,5},{2}}
		

Links

• Alois P. Heinz, Table of n, a(n) for n = 0..250

Crossrefs

For separable partitions we have A386583, sums A325534, ranks A335433.

For inseparable partitions we have A386584, sums A325535, ranks A335448.

For separable type partitions we have A386585, sums A336106, ranks A335127.

For inseparable type partitions we have A386586, sums A386638 or A025065, ranks A335126.

The complement is counted by A386633, sums of A386635.

Row sums of A386636.

A000110 counts set partitions, row sums of A048993.

A000670 counts ordered set partitions.

A003242 and A335452 count anti-runs, ranks A333489, patterns A005649.

A279790 counts disjoint families on strongly normal multisets.

A335434 counts separable factorizations, inseparable A333487.

A336103 counts normal separable multisets, inseparable A336102.

A386587 counts disjoint families of strict partitions of each prime exponent.

Cf. A001055, A124762, A239455, A335125, A386575, A386579.

Programs

Mathematica

sps[{}]:={{}};sps[set:{i_,_}]:=Join@@Function[s,Prepend[#,s]&/@sps[Complement[set,s]]]/@Cases[Subsets[set],{i,_}];
stnseps[stn_]:=Select[Permutations[Union@@stn],And@@Table[Position[stn,#[[i]]][[1,1]]!=Position[stn,#[[i+1]]][[1,1]],{i,Length[#]-1}]&]
Table[Length[Select[sps[Range[n]],stnseps[#]=={}&]],{n,0,5}]

Extensions

a(12)-a(29) from Alois P. Heinz, Aug 10 2025