OFFSET
1,2
LINKS
G. C. Greubel, Table of n, a(n) for n = 1..455
FORMULA
E.g.f.: -exp(2) + Sum_{n>=0} (2 + x^n)^n / n!.
a(n) = Sum_{d|n} 2^(d-n/d) * binomial(d, n/d) * n!/d! for n>=1.
EXAMPLE
E.g.f.: A(x) = x + 4*x^2/2! + 12*x^3/3! + 44*x^4/4! + 80*x^5/5! + 912*x^6/6! +...
where
A(x) = x*exp(2*x) + x^4*exp(2*x^2)/2! + x^9*exp(2*x^3)/3! + x^16*exp(2*x^4)/4! + x^25*exp(2*x^5)/5! + x^36*exp(2*x^6)/6! +...
also
A(x) = -exp(2) + 1 + (2+x) + (2+x^2)^2/2! + (2+x^3)^3/3! + (2+x^4)^4/4! + (2+x^5)^5/5! + (2+x^6)^6/6! +...
MATHEMATICA
a[n_] := DivisorSum[n, 2^(#-n/#) Binomial[#, n/#] n!/#!& ]; Array[a, 30] (* Jean-François Alcover, Dec 18 2015 *)
PROG
(PARI) {a(n) = local(A=1); A = sum(m=1, n, x^(m^2) * exp(2*x^m +x*O(x^n)) / m!); n!*polcoeff(A, n)}
for(n=1, 30, print1(a(n), ", "))
(PARI) {a(n) = local(A=1); A = -exp(2) + sum(m=0, n, (2 + x^m +x*O(x^n))^m/m!); if(n==0, 0, n!*polcoeff(A, n))}
for(n=1, 30, print1(a(n), ", "))
(PARI) {a(n) = if(n<1, 0, sumdiv(n, d, 2^(d-n/d) * binomial(d, n/d) * n!/d! ) )}
for(n=1, 30, print1(a(n), ", "))
CROSSREFS
KEYWORD
nonn
AUTHOR
Paul D. Hanna, Jun 21 2015
STATUS
approved