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 A106236 Triangle of the numbers of different forests with m rooted trees having distinct orders. 2

%I

%S 1,1,0,2,1,0,4,2,0,0,9,6,0,0,0,20,13,2,0,0,0,48,37,4,0,0,0,0,115,86,

%T 17,0,0,0,0,0,286,239,46,0,0,0,0,0,0,719,577,142,8,0,0,0,0,0,0,1842,

%U 1607,367,18,0,0,0,0,0,0,0,4766,4025,1136,76,0,0,0,0,0,0,0,0,12486,11185,2945,248,0,0,0,0,0,0,0,0,0

%N Triangle of the numbers of different forests with m rooted trees having distinct orders.

%C a(n) = 0 if and only if n < m + (((1+m)*m - 1)^2 -1)/8, where m is the number of trees in the forests counted by a(n).

%H Alois P. Heinz, <a href="/A106236/b106236.txt">Rows n = 1..141, flattened</a>

%F a(n) = sum over the partitions of N: 1K1 + 2K2 + ... + NKN, with exactly m distinct parts, of Product_{i=1..N}binomial(A000081(i)+Ki-1, Ki). Because all the multiplicities of the parts of the considered partitions are 1, or 0, we can simplify the formula to a(n)= sum over the partitions of N with exactly m distinct parts, of Product_{i=1..N}A000081(i). (Naturally, we do not consider the parts with multiplicity 0.)

%F G.f.: Product_{k>0} (1 + y*A000081(k)*x^k). - _Vladeta Jovovic_, May 14 2005

%e a(3) = 0 because m = 2 and (see comments) 3 < (2 + 3).

%e a(4) > 0 because m = 1. Note that (((1+m)*m - 1)^2 -1)/8 = 0, if m = 1. It is clear that n >= m.

%p with(numtheory):

%p d*g(d), d=divisors(j))*g(n-j), j=1..n-1))/(n-1))

%p end:

%p b:= proc(n, i) option remember; `if`(n=0, 1, `if`(i<1, 0,

%p j=0..min(1, n/i)))))

%p end:

%p T:= n-> (p-> seq(coeff(p, x, i), i=1..n))(b(n\$2)):

%p seq(T(n), n=1..14); # _Alois P. Heinz_, Jun 25 2014

%t g[n_] := g[n] = If[n <= 1, n, (Sum[Sum[d*g[d], {d, Divisors[j]}]*g[n-j], {j, 1, n-1}])/(n-1)]; b[n_, i_] := b[n, i] = If[n == 0, 1, If[i<1, 0, Expand[Sum[x^j*b[n - i*j, i-1]*Binomial[g[i]+j-1, j], {j, 0,Min[1, n/i]}]]]]; T[n_] := Function[{p}, Table[Coefficient[p, x, i], {i, 1, n}]][b[n, n]]; Table[T[n], {n, 1, 14}] // Flatten (* _Jean-François Alcover_, Feb 23 2015, after _Alois P. Heinz_ *)

%Y Cf. A000081, A106234.

%K nonn,tabl

%O 1,4

%A _Washington Bomfim_, Apr 28 2005

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Last modified August 12 11:14 EDT 2020. Contains 336438 sequences. (Running on oeis4.)