OFFSET
1,3
COMMENTS
Also number of ways to choose a sum-preserving permutation of a set partition of a strict partition of n.
LINKS
EXAMPLE
The a(6)=11 twice-partitions are:
((6)),
((5)(1)), ((51)),
((4)(2)), ((42)),
((3)(2)(1)), ((3)(21)), ((21)(3)), ((31)(2)), ((32)(1)), ((321)).
MATHEMATICA
nn=20;
sps[{}]:={{}}; sps[set:{i_, ___}]:=Join@@Function[s, Prepend[#, s]&/@sps[Complement[set, s]]]/@Cases[Subsets[set], {i, ___}];
Table[Total[Sum[Times@@Factorial/@Length/@Split[Sort[Total/@ptn]], {ptn, sps[Sort[#]]}]&/@Select[IntegerPartitions[n], UnsameQ@@#&]], {n, nn}]
CROSSREFS
KEYWORD
nonn
AUTHOR
Gus Wiseman, Dec 18 2016
STATUS
approved