OFFSET
1,2
COMMENTS
The class of all P-series is a subclass of the class of series-parallel posets and it contains the class of P-graphs as a subclass.
A poset is called a P-graph if it can be expressed as the ordinal sum of the antichain posets (including the singleton poset).
A poset is called a P-series if it is either a P-graph or it can be expressed as the direct sum of the P-graphs.
For example, all the 3-element posets are P-series, where only the connected posets and the antichains are P-graphs. On the other hand, the 4-element poset <{x,y,z,w},{x<.z, z<.w, y<.w, x||y, y||z}> and its dual are both series-parallel which are not the P-series. Here, by 'x<.z' we mean 'x is covered by z'.
LINKS
Alois P. Heinz, Table of n, a(n) for n = 1..3217
FORMULA
G.f: -1 + exp(Sum_{k>=1} B(x^k)/k) where B(x) = x*(1 - 2*x + 2*x^2)/((1 - x)*(1 - 2*x)). - Andrew Howroyd, Jan 06 2022
MAPLE
a:= proc(n) option remember; `if`(n=0, 1, add(a(n-j)*add(d*
max(1, 2^(d-1)-1), d=numtheory[divisors](j)), j=1..n)/n)
end:
seq(a(n), n=1..30); # Alois P. Heinz, Jan 05 2022
MATHEMATICA
a[n_] := a[n] = If[n == 0, 1, Sum[a[n - j]*Sum[d*
Max[1, 2^(d - 1) - 1], {d, Divisors[j]}], {j, 1, n}]/n];
Table[a[n], {n, 1, 30}] (* Jean-François Alcover, Mar 18 2022, after Alois P. Heinz *)
PROG
(PARI) EulerT(v)={Vec(exp(x*Ser(dirmul(v, vector(#v, n, 1/n))))-1, -#v)}
seq(n)={EulerT(Vec((1 -2*x +2*x^2)/((1-x)*(1-2*x)) + O(x*x^n)))} \\ Andrew Howroyd, Nov 19 2021
CROSSREFS
KEYWORD
nonn
AUTHOR
Salah Uddin Mohammad, Nov 12 2021
STATUS
approved