OFFSET
1,2
FORMULA
G.f.: Sum_{n>=1} log(1 + a(n)*x^n/n!) = Sum_{n>=1} n^(n-1)*x^n/n! = -LambertW(-x).
G.f.: Sum_{n>=1} log(1 + a(n)*exp(-n*x)*x^n/n!) = x.
From Paul D. Hanna, Apr 15 2009: (Start)
G.f.: Sum_{n>=1} n*a(n)*x^n/(n! + a(n)*x^n) = Sum_{n>=1} n^n*x^n/n!.
G.f.: Sum_{n>=1} n*a(n)*x^n/(n!*exp(nx) + a(n)*x^n) = x/(1-x).
Recurrence:
a(n) = n^(n-1) + (n-1)!*((-1)^n + Sum_{d|n, 1<d<n} d*( -a(d)/d! )^(n/d) ) for n > 1 with a(1)=1.
(End)
EXAMPLE
G.f.: W(x) = (1+x)*(1+3*x^2/2!)*(1+7*x^3/3!)*(1+97*x^4/4!)*(1+601*x^5/5!)* ...
W(x) = 1 + x + 3*x^2/2! + 4^2*x^3/3! + 5^3*x^4/4! + 6^4*x^5/5! + ...
where W(x/exp(x)) = exp(x) and exp(x*W(x)) = W(x) = LambertW(-x)/(-x).
PROG
(PARI) {a(n)=if(n<1, 0, polcoeff(sum(k=0, n, (k+1)^(k-1)*x^k/k!)/prod(k=1, n-1, 1+a(k)*x^k +x*O(x^n)), n))}
(PARI) {a(n)=if(n<1, 0, if(n==1, 1, n^(n-1) + (n-1)!*((-1)^n + sumdiv(n, d, if(d<n&d>1, d*(-a(d)/d!)^(n/d))))))} \\ Paul D. Hanna, Apr 15 2009
CROSSREFS
KEYWORD
nonn
AUTHOR
Paul D. Hanna, Apr 15 2009
STATUS
approved