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A274938
Number of unlabeled forests with n nodes that have two components, neither of which is the empty graph.
4
0, 0, 0, 1, 1, 3, 5, 11, 21, 46, 96, 216, 482, 1121, 2633, 6334, 15414, 38132, 95321, 241029, 614862, 1582030, 4099922, 10697038, 28074752, 74086468, 196469601, 523383136, 1400048426, 3759508536, 10131089877, 27391132238, 74283533023, 202030012554, 550934011491, 1506161266348
OFFSET
0,6
LINKS
FORMULA
G.f.: [A(x)^2 - A(x^2)]/2 where A(x) is the o.g.f. for A000055 without the initial constant 1.
a(2n+1) = A274937(2n+1). a(2n) = A274937(2n)-A000055(n). - R. J. Mathar, Jul 20 2016
MAPLE
with(numtheory):
b:= proc(n) option remember; `if`(n<2, n, (add(add(d*
b(d), d=divisors(j))*b(n-j), j=1..n-1))/(n-1))
end:
g:= proc(n) option remember; `if`(n=0, 1, b(n)-add(b(j)*
b(n-j), j=0..n/2)+`if`(n::odd, 0, (t->t*(t+1)/2)(b(n/2))))
end:
a:= proc(n) option remember; add(g(j)*g(n-j), j=1..n/2)-
`if`(n::odd or n=0, 0, (t-> t*(t+1)/2)(g(n/2)))
end:
seq(a(n), n=0..40); # Alois P. Heinz, Jul 20 2016
MATHEMATICA
b[n_] := b[n] = If[n<2, n, Sum[DivisorSum[j, #*b[#]&]*b[n-j], {j, 1, n-1}]/(n-1)];
g[n_] := g[n] = If[n==0, 1, b[n]-Sum[b[j]*b[n-j], {j, 0, n/2}]+If[OddQ[n], 0, Function[t, t*(t+1)/2][b[n/2]]]];
a[n_] := a[n] = Sum[g[j]*g[n-j], {j, 1, n/2}]-If[OddQ[n] || n==0, 0, Function[t, t*(t+1)/2][g[n/2]]];
Table[a[n], {n, 0, 40}] (* Jean-François Alcover, Mar 15 2017, after Alois P. Heinz *)
CROSSREFS
Cf. A000055, A274935-A274937. [A274935, A274936, A274937, A274938] are analogs for forests of [A275165, A275166, A216785, A274934] for graphs.
Sequence in context: A283905 A146574 A007873 * A283603 A283818 A352006
KEYWORD
nonn
AUTHOR
N. J. A. Sloane, Jul 19 2016
STATUS
approved