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 A181360 Number of forests of rooted trees containing n nodes not counting the root nodes. 3
 1, 1, 3, 7, 19, 47, 127, 330, 889, 2378, 6450, 17510, 47907, 131388, 362081, 1000665, 2774857, 7714695, 21505455, 60084062, 168234804, 471977022, 1326558625, 3734804268, 10531738149, 29742332548, 84111212892, 238176473946, 675269414372, 1916715819186 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 COMMENTS Every tree in the forest must have at least 2 nodes, i.e. at least one more node besides the root. - N. J. A. Sloane, Nov 26 2010 First, T(n), the number of rooted trees with n+1 nodes A000081(n+1) can be computed using partitions of n as follows: let n = (q1*1 + q2*2 + q3*3 + ... + qn*n) be a nonnegative integer partition of n (the "q"s are the multiplicities of the part sizes), and define a^b to be (a+b-1)! / (a-1)! / b! (the number of ways to color b identical items with a colors), then compute the sum of T(0)^q1 * T(1)^q2 * ... * T(n-1)^qn over all such partitions of n. Then F(n), the number of forests of rooted trees containing N nodes not counting the roots, can be similarly computed as the sum of T(1)^q1 * T(2)^q2 * ... * T(n)^qn over all such partitions of n. These are the diagonal sums of the triangle in A174135. - N. J. A. Sloane, Nov 26 2010. LINKS N. J. A. Sloane and Alois P. Heinz, Table of n, a(n) for n = 0..2133 A. Mansuy, Preordered forests, packed words and contraction algebras, J. Algebra 411 (2014) 259-311, section 4.1 R. J. Mathar, Topologically Distinct Sets of Non-intersecting Circles in the Plane, arXiv:1603.00077 [math.CO], 2016. FORMULA a(n) ~ c * d^n / n^(3/2), where d = 2.955765285651994974714817524... is the Otter's rooted tree constant (see A051491), and c = 10.088029891871277227771831767... . - Vaclav Kotesovec, May 09 2014 a(n) = A033185(2n, n). - Alois P. Heinz, Feb 15 2016 a(n) = A033185(2n+k, n+k) for all n, k >= 0. - Michael Somos, Aug 20 2018 EXAMPLE Trees for example (leaving out the "^0" factors for clarity): T(0) = 1, T(1) = 1 T(2) = T(1)^1 + T(0)^2 = 2, T(3) = T(2)^1 + T(1)^1*T(0)^1 + T(0)^3 = 4, T(4) = T(3)^1 + T(2)^1*T(0)^1 + T(1)^2 + T(1)^1*T(0)^2 +T(0)^4 = 9, T(5) = T(4)^1 + T(3)^1*T(0)^1 + T(2)^1*T(1)^1 + T(2)^1*T(0)^2 + T(1)^2*T(0)^1 + T(1)^1*T(0)^3 + T(0)^5 = 20. Forests for example (leaving out the "^0" factors for clarity): F(2) = T(2)^1 + T(1)^2 = 3, F(3) = T(3)^1 + T(2)^1*T(1)^1 + T(1)^3 = 7, F(4) = T(4)^1 + T(3)^1*T(1)^1 + T(2)^2 + T(2)*T(1)^2 + T(1)^4 = 19, F(5) = T(5)^1 + T(4)^1*T(1)^1 + T(3)^1*T(2)^1 + T(3)^1*T(1)^2 + T(2)^2*T(1)^1 + T(2)^1*T(1)^3 + T(1)^5 = 47. {Examples of this a^b definition: 2^1 = 2, 2^2 = 3, 2^3 = 4, 2^4 = 5, 3^1 = 3, 3^2 = 6, 3^3 = 10, 3^4 = 15, (triangular numbers) 4^1 = 4, 4^2 = 10, 4^3 = 20, 4^4 = 35, (tetrahedral numbers) equivalently a^b = (b == 0 ? 1 : (a == 1 || b == 1 ? a : (a * (a+1)^(b-1) / b))) } MAPLE (From N. J. A. Sloane, Nov 26 2010) First read 110 terms of A000081 into array b1 M:=100; t1:=1/mul((1-x*y^i)^b1[i+1], i=2..M): t2:=series(t1, y, M): t3:=series(t2, x, M): a:=(n, k)->coeff(coeff(t3, x, k), y, n); g:=n->add(a(n-1+i, i), i=1..n-1); [seq(g(n), n=1..48)]; # second Maple program: g:= proc(n, i) option remember; `if`(n=0, 1, `if`(i<1, 0,       add(binomial(T(i-1)+j-1, j) *g(n-i*j, i-1), j=0..n/i)))     end: T:= n-> g(n, n): b:= proc(n, i) option remember; `if`(n=0, 1, `if`(i<1, 0,       add(binomial(T(i)+j-1, j) *b(n-i*j, i-1), j=0..n/i)))     end: a:= n-> b(n, n): seq(a(n), n=0..40);  # Alois P. Heinz, Jul 20 2012 # third Maple program: g:= proc(n) option remember; `if`(n<=1, n, (add(add(d*       g(d), d=numtheory[divisors](j))*g(n-j), j=1..n-1))/(n-1))     end: a:= proc(n) option remember; `if`(n=0, 1, add(add(d*       g(d+1), d=numtheory[divisors](j))*a(n-j), j=1..n)/n)     end: seq(a(n), n=0..40);  # Alois P. Heinz, Sep 19 2017 MATHEMATICA g[n_, i_] := g[n, i] = If[n==0, 1, If[i<1, 0, Sum[Binomial[T[i-1]+j-1, j]*g[n-i*j, i-1], {j, 0, n/i}]]]; T[n_] := g[n, n]; b[n_, i_] := b[n, i] = If[n==0, 1, If[i<1, 0, Sum[Binomial[T[i]+j-1, j]*b[n-i*j, i-1], {j, 0, n/i}]]]; a[n_] := b[n, n] // FullSimplify; Table[a[n], {n, 1, 40}] (* Jean-François Alcover, Mar 30 2015, after Alois P. Heinz *) CROSSREFS Cf. A000081 (rooted trees). Cf. A093637 (products of partition numbers). Cf. A174135, A033185. Sequence in context: A110014 A026581 A151535 * A001372 A179467 A308153 Adjacent sequences:  A181357 A181358 A181359 * A181361 A181362 A181363 KEYWORD nonn AUTHOR Peter A. Lawrence, Oct 15 2010 EXTENSIONS a(0)=1 prepended by Alois P. Heinz, Sep 19 2017 STATUS approved

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Last modified October 19 15:34 EDT 2021. Contains 348091 sequences. (Running on oeis4.)