OFFSET
1,2
FORMULA
a(n) = n*(n-2)!* Sum_{k=1..n-1} C(n-1,k-1)* a(k)/k! for n>1 with a(1)=1.
a(n) = n*A193161(n-1).
EXAMPLE
E.g.f.: A(x) = x + 2*x^2/2! + 9*x^3/3! + 68*x^4/4! + 760*x^5/5! +...
Related expansions:
A(x/(1-x)) = x + 4*x^2/2! + 27*x^3/3! + 272*x^4/4! + 3800*x^5/5! +...
x*A'(x) = x + 4*x^2/2! + 27*x^3/3! + 272*x^4/4! + 3800*x^5/5! +...
PROG
(PARI) {a(n)=local(A=[1], F=x); for(i=1, n, A=concat(A, 0); F=x*Ser(A); A[#A]=Vec(subst(F, x, x/(1-x)))[#A]/(#A-1)); if(n<1, 0, n!*A[n])}
(PARI) {a(n)=if(n<1, 0, if(n==1, 1, n!/(n-1)*sum(k=1, n-1, binomial(n-1, k-1)*a(k)/k!)))}
CROSSREFS
KEYWORD
nonn
AUTHOR
Paul D. Hanna, Jul 16 2011
STATUS
approved