OFFSET
1,2
COMMENTS
Also number of labeled rooted trees with n generators. (A generator is a leaf or a node with just one child.) - Christian G. Bower, Jun 07 2005
REFERENCES
L. R. Foulds and R. W. Robinson, Counting certain classes of evolutionary trees with singleton labels, Congress. Num., 44 (1984), 65-88.
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
LINKS
G. C. Greubel, Table of n, a(n) for n = 1..360
L. R. Foulds and R. W. Robinson, Counting certain classes of evolutionary trees with singleton labels, Congress. Num., 44 (1984), 65-88. (Annotated scanned copy)
FORMULA
E.g.f. satisfies (2-x)*A(x) = x - 1 + exp(A(x)). - Christian G. Bower, Jun 07 2005
a(n) = Sum_{k=1..(n-1)} (n+k-1)!*Sum_{j=1..k} 1/((k-j)!)*Sum_{i=0..(n-1)} binomial(j+i-1,j-1)*Sum_{m=0..j} 2^m*(-1)^(m+i)*Stirling2(n-m+j-i-1,j-m))/(m!*(n-m+j-i-1)!), n>1, a(1)=1. - Vladimir Kruchinin, Aug 07 2012
a(n) ~ sqrt(LambertW(1)+1) * n^(n-1) * (LambertW(1))^n / (exp(n) * (2*LambertW(1)-1)^(n-1/2)). - Vaclav Kotesovec, Jan 08 2014
MAPLE
A007151 := proc(n)
local k, j, i, m , a;
if n =1 then
1;
else
a := 0 ;
for k from 1 to n-1 do
for j from 1 to k do
for i from 0 to n-1 do
for m from 0 to j do
a := a+(n+k-1)! /(k-j)! *binomial(j+i-1, j-1) *2^m *(-1)^(m+i) *combinat[stirling2](n-m+j-i-1, j-m) / m! /(n-m+j-i-1)! ;
end do:
end do:
end do:
end do:
a ;
end if;
end proc:
seq(A007151(n), n=1..10) ; # R. J. Mathar, Mar 19 2018
MATHEMATICA
Rest[CoefficientList[InverseSeries[Series[(1 - E^x + 2*x)/(1 + x), {x, 0, 20}], x], x] * Range[0, 20]!] (* Vaclav Kotesovec, Jan 08 2014 *)
PROG
(Maxima) a(n):=if n=1 then 1 else (sum((n+k-1)!*sum(1/((k-j)!)*sum(binomial(j+i-1, j-1)*sum((2^m*(-1)^(m+i)*stirling2(n-m+j-i-1, j-m))/(m!*(n-m+j-i-1)!), m, 0, j), i, 0, n-1), j, 1, k), k, 1, n-1)); /* Vladimir Kruchinin, Aug 07 2012 */
(PARI) for(n=1, 20, print1(if(n==1, 1, sum(k=1, n-1, (n+k-1)!*sum(j=1, k, (1/(k-j)!)* sum(i=0, n-1, binomial(j+i-1, j-1)*sum(m=0, j, 2^m*(-1)^(m+i)* stirling(n-m+j-i-1, j-m, 2)/(m!*(n-m+j-i-1)!)))))), ", ")) \\ G. C. Greubel, Nov 26 2017
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
STATUS
approved