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A326268 E.g.f.: Sum_{n>=0} (1 + W(x)^n)^n * x^n / n!, where W(x) = exp(x*W(x)) = LambertW(-x)/(-x). 2
1, 2, 6, 41, 464, 7137, 138520, 3262429, 90838256, 2933881793, 108328840784, 4520094828933, 211121218481464, 10950494124192625, 626447138747705384, 39291583224925510373, 2687826475163234708960, 199600950459114370987905, 16023820242226719843186976, 1385388282982694845147725925, 128555684722289147339542911656, 12763504615760744636458361018417, 1351971567374693190451022777333816 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

COMMENTS

More generally, the following sums are equal:

(1) Sum_{n>=0} (p + q^n)^n * r^n/n!,

(2) Sum_{n>=0} q^(n^2) * exp(p*q^n*r) * r^n/n!;

here, q = LambertW(-x)/(-x) with p = 1, r = x.

LINKS

Table of n, a(n) for n=0..22.

FORMULA

Let W(x) = LambertW(-x)/(-x), then e.g.f. A(x) equals the following sums.

(1) Sum_{n>=0} (1 + W(x)^n)^n * x^n / n!.

(2) Sum_{n>=0} W(x)^(n^2) * exp( W(x)^n * x ) / n!.

EXAMPLE

E.g.f.: A(x) = 1 + 2*x + 6*x^2/2! + 41*x^3/3! + 464*x^4/4! + 7137*x^5/5! + 138520*x^6/6! + 3262429*x^7/7! + 90838256*x^8/8! + 2933881793*x^9/9! + 108328840784*x^10/10! + ...

such that

A(x) = 1 + (1 + W(x))*x + (1 + W(x)^2)^2*x^2/2! + (1 + W(x)^3)^3*x^3/3! + (1 + W(x)^4)^4*x^4/4! + (1 + W(x)^5)^5*x^5/5! + (1 + W(x)^6)^6*x^6/6! + (1 + W(x)^7)^7*x^7/7! + (1 + W(x)^8)^8*x^8/8! + ...

also

A(x) = exp(x) + W(x)*exp(W(x)*x)*x + W(x)^4*exp(W(x)^2*x)*x^2/2! + W(x)^9*exp(W(x)^3*x)*x^3/3! + W(x)^16*exp(W(x)^4*x)*x^4/4! + W(x)^25*exp(W(x)^5*x)*x^5/5! + W(x)^36*exp(W(x)^6*x)*x^6/6! + ...

where W(x) = exp(x*W(x)) = LambertW(-x)/(-x) begins

W(x) = 1 + x + 3*x^2/2! + 16*x^3/3! + 125*x^4/4! + 1296*x^5/5! + 16807*x^6/6! + 262144*x^7/7! + 4782969*x^8/8! + 100000000*x^9/9! + ... + (n+1)^(n-1)*x^n/n! + ...

RELATED SERIES.

Note that W(x)^n equals

W(x)^n = Sum_{k>=0} n * (n + k)^(k-1) * x^k/k!

and so

W(x)^(n^2) = Sum_{k>=0} n^2 * (n^2 + k)^(k-1) * x^k/k!.

PROG

(PARI) /* E.g.f.: Sum_{n>=0} (1 + W(x)^n)^n * x^n / n! */

{a(n) = my(W = 1/x*serreverse(x*exp(-x +x*O(x^n))));

n! * polcoeff( sum(m=0, n, (1 + W^m)^m * x^m / m!), n)}

for(n=0, 30, print1(a(n), ", "))

(PARI) /* E.g.f.: Sum_{n>=0} W(x)^(n^2) * exp( -W(x)^n * x ) / n! */

{a(n) = my(W = 1/x*serreverse(x*exp(-x +x*O(x^n))));

n! * polcoeff( sum(m=0, n, W^(m^2) * exp(W^m*x +x*O(x^n)) * x^m / m!), n)}

for(n=0, 30, print1(a(n), ", "))

CROSSREFS

Cf. A326266, A326267.

Sequence in context: A118623 A000612 A319633 * A096138 A004153 A071440

Adjacent sequences:  A326265 A326266 A326267 * A326269 A326270 A326271

KEYWORD

nonn

AUTHOR

Paul D. Hanna, Jun 29 2019

STATUS

approved

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Last modified March 28 07:59 EDT 2020. Contains 333079 sequences. (Running on oeis4.)