OFFSET
0,3
COMMENTS
A multiset partition is normal if it covers an initial interval of positive integers. It is sortable if some permutation has an ordered concatenation. For example, the multiset partition {{1,2},{1,1,1},{2,2,2}} is sortable because the permutation ((1,1,1),(1,2),(2,2,2)) has concatenation (1,1,1,1,2,2,2,2), which is weakly increasing.
LINKS
Andrew Howroyd, Table of n, a(n) for n = 0..1000
FORMULA
G.f.: ((1 - x)*(1 - 2*x) - x^2*P(x))/(2*(1 - x)*(1 - 2*x) - (1 - 3*x + 4*x^2)*P(x)) where P(x) is the g.f. of A000041. - Andrew Howroyd, May 11 2023
EXAMPLE
The a(0) = 1 through a(3) = 15 multiset partitions:
{} {{1}} {{1,1}} {{1,1,1}}
{{1,2}} {{1,1,2}}
{{1},{1}} {{1,2,2}}
{{1},{2}} {{1,2,3}}
{{1},{1,1}}
{{1},{1,2}}
{{1,1},{2}}
{{1},{2,2}}
{{1,2},{2}}
{{1},{2,3}}
{{1,2},{3}}
{{1},{1},{1}}
{{1},{1},{2}}
{{1},{2},{2}}
{{1},{2},{3}}
MATHEMATICA
lexsort[f_, c_]:=OrderedQ[PadRight[{f, c}]];
allnorm[n_]:=If[n<=0, {{}}, Function[s, Array[Count[s, y_/; y<=#]+1&, n]]/@Subsets[Range[n-1]+1]];
sps[{}]:={{}}; sps[set:{i_, ___}]:=Join@@Function[s, Prepend[#, s]&/@sps[Complement[set, s]]]/@Cases[Subsets[set], {i, ___}];
mps[set_]:=Union[Sort[Sort/@(#/.x_Integer:>set[[x]])]&/@sps[Range[Length[set]]]];
Table[Length[Select[Sort[#, lexsort]&/@Join@@mps/@allnorm[n], OrderedQ[Join@@#]&]], {n, 0, 5}]
PROG
(PARI) seq(n) = my(p=1/eta(x + O(x*x^n))); Vec(((1 - x)*(1 - 2*x) - x^2*p)/(2*(1 - x)*(1 - 2*x) - (1 - 3*x + 4*x^2)*p)) \\ Andrew Howroyd, May 11 2023
CROSSREFS
KEYWORD
nonn
AUTHOR
Gus Wiseman, Jun 19 2019
EXTENSIONS
Terms a(10) and beyond from Andrew Howroyd, May 11 2023
STATUS
approved