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 A005196 a(n) = Sum_t t*F(n,t), where F(n,t) (see A095133) is the number of forests with n (unlabeled) nodes and exactly t trees. (Formerly M2567) 6
 1, 3, 6, 13, 24, 49, 93, 190, 381, 803, 1703, 3755, 8401, 19338, 45275, 108229, 262604, 647083, 1613941, 4072198, 10374138, 26663390, 69056163, 180098668, 472604314, 1247159936, 3307845730, 8814122981, 23585720703, 63359160443, 170815541708, 462049250165 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 REFERENCES N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). LINKS Alois P. Heinz, Table of n, a(n) for n = 1..300 E. M. Palmer and A. J. Schwenk, On the number of trees in a random forest, J. Combin. Theory, B 27 (1979), 109-121. [The 17th entry is wrong] Eric Weisstein's World of Mathematics, Forest FORMULA To get a(n), take row n of the triangle in A095133, multiply successive terms by 1, 2, 3, ... and sum. E.g., a(4) = 1*2 + 2*2 + 3*1 + 4*1 = 13. MAPLE with(numtheory): b:= proc(n) option remember; local d, j; `if` (n<=1, n,       (add(add(d*b(d), d=divisors(j))*b(n-j), j=1..n-1))/(n-1))     end: t:= proc(n) option remember; local k; `if` (n=0, 1,       b(n)-(add(b(k)*b(n-k), k=0..n)-`if`(irem(n, 2)=0, b(n/2), 0))/2)     end: g:= proc(n, i, p) option remember; `if`(p>n, 0, `if`(n=0, 1,       `if`(min(i, p)<1, 0, add(g(n-i*j, i-1, p-j) *        binomial(t(i)+j-1, j), j=0..min(n/i, p)))))     end: a:= n-> add(k*g(n, n, k), k=1..n): seq(a(n), n=1..40);  # Alois P. Heinz, Aug 20 2012 MATHEMATICA nn=30; s[n_, k_]:=s[n, k]=a[n+1-k]+If[n<2k, 0, s[n-k, k]]; a[1]=1; a[n_]:=a[n]=Sum[a[i]s[n-1, i]i, {i, 1, n-1}]/(n-1); ft=Table[a[i]-Sum[a[j]a[i-j], {j, 1, i/2}]+If[OddQ[i], 0, a[i/2](a[i/2]+1)/2], {i, 1, nn}]; CoefficientList[Series[D[Product[1/(1-y x^i)^ft[[i]], {i, 1, nn}], y]/.y->1, {x, 0, 20}], x]  (* Geoffrey Critzer, Oct 13 2012, after code given by Robert A. Russell in A000055 *) CROSSREFS Cf. A000055, A005195, A095133. Sequence in context: A000219 A191782 A027999 * A320286 A032287 A199403 Adjacent sequences:  A005193 A005194 A005195 * A005197 A005198 A005199 KEYWORD nonn,nice AUTHOR EXTENSIONS More terms from Vladeta Jovovic, Jun 03 2004 Definition clarified by N. J. A. Sloane, May 29 2012 STATUS approved

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Last modified April 12 22:03 EDT 2021. Contains 342933 sequences. (Running on oeis4.)