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A001858 Number of forests of trees on n labeled nodes.
(Formerly M1804 N0714)

%I M1804 N0714

%S 1,1,2,7,38,291,2932,36961,561948,10026505,205608536,4767440679,

%T 123373203208,3525630110107,110284283006640,3748357699560961,

%U 137557910094840848,5421179050350334929,228359487335194570528,10239206473040881277575,486909744862576654283616

%N Number of forests of trees on n labeled nodes.

%C The number of integer lattice points in the permutation polytope of {1,2,...,n}. - _Max Alekseyev_, Jan 26 2010

%D B. Bollobas, Modern Graph Theory, Springer, 1998, p. 290.

%D N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).

%D N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

%H Alois P. Heinz, <a href="/A001858/b001858.txt">Table of n, a(n) for n = 0..388</a> (first 101 terms from T. D. Noe)

%H David Callan, <a href="https://cs.uwaterloo.ca/journals/JIS/VOL6/Callan/callan10.html">A Combinatorial Derivation of the Number of Labeled Forests</a>, J. Integer Seqs., Vol. 6, 2003.

%H Huantian Cao, <a href="http://www.cs.uga.edu/~rwr/STUDENTS/hcao.html">AutoGF: An Automated System to Calculate Coefficients of Generating Functions</a>.

%H P. Flajolet and R. Sedgewick, <a href="http://algo.inria.fr/flajolet/Publications/books.html">Analytic Combinatorics</a>, 2009; see page 132

%H Samuele Giraudo, <a href="http://igm.univ-mlv.fr/~giraudo/Articles/CliqueFPSAC.pdf">Combalgebraic structures on decorated cliques</a>, Formal Power Series and Algebraic Combinatorics, Séminaire Lotharingien de Combinatoire, 78B.15, 2017, p. 7.

%H Anton Izosimov, <a href="http://arxiv.org/abs/1506.05179">Matrix polynomials, generalized Jacobians, and graphical zonotopes</a>, arXiv preprint arXiv:1506.05179 [math.AG], 2015.

%H Arun P. Mani and Rebecca J. Stones, <a href="https://www.emis.de/journals/INTEGERS/papers/q17/q17.Abstract.html">Congruences for weighted number of labeled forests</a>, INTEGERS 16 (2016). #A17.

%H J. Pitman, <a href="http://www.stat.berkeley.edu/users/pitman/457.pdf">Coalescent Random Forests</a>, J. Combin. Theory, A85 (1999), 165-193.

%H J. Riordan, <a href="http://dx.doi.org/10.1016/S0021-9800(68)80033-X">Forests of labeled trees</a>, J. Combin. Theory, 5 (1968), 90-103.

%H J. Riordan, <a href="/A001861/a001861.pdf">Letter to N. J. A. Sloane, Oct 19 1970</a>

%H J. Riordan, <a href="/A000262/a000262_1.pdf">Letter to N. J. A. Sloane, Nov 10 1975</a>

%H John Riordan and N. J. A. Sloane, <a href="/A003471/a003471_1.pdf">Correspondence, 1974</a>

%H N. J. A. Sloane, <a href="/A001514/a001514.pdf">Letter to J. Riordan, Nov. 1970</a>

%H L. Takacs, <a href="http://dx.doi.org/10.1137/0403050">On the number of distinct forests</a>, SIAM J. Discrete Math., 3 (1990), 574-581.

%H E. M. Wright, <a href="http://plms.oxfordjournals.org/content/s3-17/2/296.extract">A relationship between two sequences</a>, Proc. London Math. Soc. (3) 17 (1967) 296-304.

%F E.g.f.: exp( Sum_{n>=1} n^(n-2)*x^n/n! ). This implies (by a theorem of Wright) that a(n) ~ exp(1/2)*n^(n-2). - _N. J. A. Sloane_, May 12 2008 [Corrected by Philippe Flajolet, Aug 17 2008]

%F E.g.f.: exp(T - T^2/2), where T = T(x) = Sum_{ n>=1} n^(n-1)*x^n/n! is Euler's tree function (see A000169). - _Len Smiley_, Dec 12 2001

%F Shifts 1 place left under the hyperbinomial transform (cf. A088956). - _Paul D. Hanna_, Nov 03 2003

%F a(0) = 1, a(n) = Sum_{j=0..n-1} C(n-1,j) (j+1)^(j-1) a(n-1-j) if n>0. - _Alois P. Heinz_, Sep 15 2008

%p exp(x+x^2+add(n^(n-2)*x^n/n!, n=3..50));

%p # second Maple program:

%p a:= proc(n) option remember; `if`(n=0, 1, add(

%p binomial(n-1, j-1)*j^(j-2)*a(n-j), j=1..n))

%p end:

%p seq(a(n), n=0..20); # _Alois P. Heinz_, Sep 15 2008

%p # third Maple program:

%p F:= exp(-LambertW(-x)*(1+LambertW(-x)/2)):

%p S:= series(F,x,51):

%p seq(coeff(S,x,j)*j!, j=0..50); # _Robert Israel_, May 21 2015

%t nn=20;t=Sum[n^(n-1)x^n/n!,{n,1,nn}];Range[0,nn]!CoefficientList[ Series[Exp[t-t^2/2],{x,0,nn}],x] (* _Geoffrey Critzer_, Sep 05 2012 *)

%o (PARI) a(n)=if(n<0,0,sum(m=0,n,sum(j=0,m,binomial(m,j)*binomial(n-1,n-m-j)*n^(n-m-j)*(m+j)!/(-2)^j)/m!)) /* _Michael Somos_, Aug 22 2002 */

%Y Cf. A088956. Row sums of A138464. Cf. also A006572, A006573.

%K nonn,easy,eigen

%O 0,3

%A _N. J. A. Sloane_ and _Simon Plouffe_

%E More terms from _Michael Somos_, Aug 22 2002

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Last modified February 22 21:55 EST 2018. Contains 299469 sequences. (Running on oeis4.)