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A215881 Expansion of e.g.f.: sqrt( -LambertW(-x) / LambertW(x) ). 6
1, 1, 1, 10, 37, 716, 4741, 136760, 1314377, 50468752, 637409641, 30580648352, 479025538861, 27578021183168, 515932095998957, 34657964676194176, 754078761294069649, 57902855910383448320, 1436649321508321044817, 124128617507138965088768, 3459197142121422461242421 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,4

LINKS

G. C. Greubel, Table of n, a(n) for n = 0..385

FORMULA

E.g.f.: exp( Sum_{n>=0} (2*n+1)^(2*n) * x^(2*n+1)/(2*n+1)! ).

a(n) = Sum_{k=0..n} -(-1)^k*C(n,k) * (k - 1/2)^(k-1) * (n-k + 1/2)^(n-k-1) / 4.

a(n) ~ c * n^(n-1), where c = 1/2*(1-LambertW(exp(-1))) / sqrt(LambertW(exp(-1))) = 0.6836640292259232... if n is even and c = 1/2*(1+LambertW(exp(-1))) / sqrt(LambertW(exp(-1))) = 1.2113614261884947... if n is odd. - Vaclav Kotesovec, Nov 27 2012

EXAMPLE

E.g.f.: A(x) = 1 + x + x^2/2! + 10*x^3/3! + 37*x^4/4! + 716*x^5/5! + 4741*x^6/6! +...

such that A(x) = sqrt( -LambertW(-x)/LambertW(x) ) where

LambertW(x) = x - 2*x^2/2! + 9*x^3/3! - 64*x^4/4! + 625*x^5/5! - 7776*x^6/6! + 117649*x^7/7! - 2097152*x^8/8! +...+ (-n)^(n-1)*x^n/n! +...

Related expansions:

log(A(x)) = x + 9*x^3/3! + 625*x^5/5! + 117649*x^7/7! + 43046721*x^9/9! +...+ (2*n-1)^(2*n-2)*x^(2*n-1)/(2*n-1)! +...

MAPLE

a:=series(sqrt(-LambertW(-x)/LambertW(x)), x=0, 21): seq(n!*coeff(a, x, n), n=0..20); # Paolo P. Lava, Mar 27 2019

MATHEMATICA

CoefficientList[Series[Sqrt[-LambertW[-x]/LambertW[x]], {x, 0, 20}], x]* Range[0, 20]! (* Vaclav Kotesovec, Nov 27 2012 *)

PROG

(PARI) {a(n)=local(LW=sum(m=1, n+1, -(-1)^m*m^(m-1)*x^m/m!)+x^2*O(x^n)); n!*polcoeff(sqrt(-subst(LW, x, -x)/LW), n)}

(PARI) {a(n)=n!*polcoeff(exp(sum(m=0, n, (2*m+1)^(2*m)*x^(2*m+1)/(2*m+1)!)+x*O(x^n)), n)}

(PARI) {a(n)=sum(k=0, n, -(-1)^k*binomial(n, k)*(k-1/2)^(k-1)*(n-k+1/2)^(n-k-1)/4)}

for(n=0, 21, print1(a(n), ", "))

(PARI) x='x+O('x^30); Vec(serlaplace(sqrt(-lambertw(-x)/lambertw(x)))) \\ G. C. Greubel, Feb 19 2018

(GAP) List([0..25], n->Sum([0..n], k->-(-1)^k*Binomial(n, k)*(k-(1/2))^(k-1)*(n-k+(1/2))^(n-k-1)/4)); # Muniru A Asiru, Feb 19 2018

CROSSREFS

Cf. A215880, A215882, A215890, A138734, A216143.

Sequence in context: A137280 A071261 A129426 * A279543 A065009 A031430

Adjacent sequences:  A215878 A215879 A215880 * A215882 A215883 A215884

KEYWORD

nonn

AUTHOR

Paul D. Hanna, Aug 25 2012

STATUS

approved

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Last modified October 22 19:55 EDT 2020. Contains 337954 sequences. (Running on oeis4.)