A215882 -id:A215882 - cp's OEIS frontend

A215880 Expansion of e.g.f.: sqrt( -LambertW(x)*LambertW(-x)/x^2 ).

1, 0, 2, 0, 76, 0, 9816, 0, 2731408, 0, 1327394080, 0, 998915378880, 0, 1076892609950080, 0, 1575062807865569536, 0, 3002359129582191616512, 0, 7232182908954405203184640, 0, 21483641086491531479896545280, 0, 77155816749532379835853517131776, 0

Offset: 0

Paul D. Hanna, Aug 25 2012

nonn

			E.g.f.: A(x) = 1 + 2*x^2/2! + 76*x^4/4! + 9816*x^6/6! + 2731408*x^8/8! + ...
such that A(x) = sqrt( -LambertW(x)*LambertW(-x)/x^2 ) 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:
A(x)^2 = 1 + 4*x^2/2! + 176*x^4/4! + 24192*x^6/6! + 6966528*x^8/8! + 3459768320*x^10/10! + ... + A138734(n)*x^n/n! + ...
log(A(x)) = 2*x^2/2! + 64*x^4/4! + 7776*x^6/6! + 2097152*x^8/8! + 1000000000*x^10/10! + ... + (2*n)^(2*n-1)*x^(2*n)/(2*n)! + ...

Vincenzo Librandi, Table of n, a(n) for n = 0..200

Cf. A215881, A215882, A138734, A216143, A215890, A216409.

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

CoefficientList[Series[Sqrt[-LambertW[x]*LambertW[-x]/x^2], {x, 0, 20}], x]* Range[0, 20]! (* Vaclav Kotesovec, Nov 27 2012 *)

{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(-LW*subst(LW, x, -x)/x^2), n)}

{a(n)=local(Chw=sum(m=0, n\2, (2*m+1)^(2*m-1)*x^(2*m)/(2*m)!)+x*O(x^n),Shw=sum(m=0, n\2, (2*m+2)^(2*m)*x^(2*m+1)/(2*m+1)!)+x*O(x^n)); n!*polcoeff(sqrt(Chw^2-Shw^2), n)}

/* E.g.f.: sqrt(Chw(x,t)^2 - Shw(x,t)^2)^(1/t) for any |t|>0: */
{a(n)=local(Chw_t=sum(m=0, n\2, t*(2*m+t)^(2*m-1)*x^(2*m)/(2*m)!)+x*O(x^n),
Shw_t=sum(m=0, n\2, t*(2*m+t+1)^(2*m)*x^(2*m+1)/(2*m+1)!)+x*O(x^n));
n!*polcoeff(sqrt(Chw_t^2-Shw_t^2)^(1/t), n)}

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

{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),", "))

x='x+O('x^30); Vec(serlaplace(sqrt(-lambertw(x)*lambertw(-x)/ x^2))) \\ G. C. Greubel, Feb 19 2018

E.g.f.: exp( Sum_{n>=1} (2*n)^(2*n-1) * x^(2*n)/(2*n)! ).
E.g.f.: sqrt( Chw(x)^2 - Shw(x)^2 ), where
Chw(x) = Sum_{n>=0} (2*n+1)^(2*n-1) * x^(2*n)/(2*n)! and
Shw(x) = Sum_{n>=0} (2*n+2)^(2*n) * x^(2*n+1)/(2*n+1)!.
E.g.f.: sqrt( Chw(x,t)^2 - Shw(x,t)^2 )^(1/t) for |t|>0, where
Chw(x,t) = Sum_{n>=0} t*(2*n+t)^(2*n-1) * x^(2*n)/(2*n)! and
Shw(x,t) = Sum_{n>=0} t*(2*n+t+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.
If n is even, a(n) ~ exp(1)*sqrt(LambertW(exp(-1)))*n^(n-1) = 1.434430245088497558... * n^(n-1). - Vaclav Kotesovec, Nov 27 2012

A215881 Expansion of e.g.f.: sqrt( -LambertW(-x) / LambertW(x) ).

Original entry on oeis.org

1, 1, 1, 10, 37, 716, 4741, 136760, 1314377, 50468752, 637409641, 30580648352, 479025538861, 27578021183168, 515932095998957, 34657964676194176, 754078761294069649, 57902855910383448320, 1436649321508321044817, 124128617507138965088768, 3459197142121422461242421

Offset: 0

Paul D. Hanna, Aug 25 2012

nonn

			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)! +...

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

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

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

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

{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)}

{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)}

{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),", "))

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

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

A215890 E.g.f.: Shw(x) / Chw(x), where Chw(x) = Sum_{n>=0} (2n+1)^(2n-1) * x^(2n)/(2n)! and Shw(x) = Sum_{n>=0} (2n+2)^(2n) * x^(2n+1)/(2n+1)!, with zero coefficients omitted.

Original entry on oeis.org

1, 7, 461, 84827, 30648697, 18319873199, 16364889804485, 20422587185959363, 33936856584735107441, 72442485189323515166807, 193177550094285124372564285, 629325007854898466696826469931, 2459394825443160188158170494692841, 11355850594904678701281528241280254207, 61160283986030409241528644478489460162357, 380011322281573634548774673447708463134715539

Offset: 1

Paul D. Hanna, Aug 25 2012

nonn

It appears that terms are not divisible by 2 or 3.

			E.g.f.: A(x) = x + 7*x^3/3! + 461*x^5/5! + 84827*x^7/7! + 30648697*x^9/9! +...
such that A(x) = Shw(x)/Chw(x) = tanh(x*Chw(x)) where
Shw(x) = x + 16*x^3/3! + 1296*x^5/5! + 262144*x^7/7! + 100000000*x^9/9! +...+ (2*n+2)^(2*n)*x^(2*n+1)/(2*n+1)! +...
Chw(x) = 1 + 3*x^2/2! + 125*x^4/4! + 16807*x^6/6! + 4782969*x^8/8! + 2357947691*x^10/10! +...+ (2*n+1)^(2*n-1)*x^(2*n)/(2*n)! +...
and Chw(x) + Shw(x) = LambertW(-x)/(-x).

Paul D. Hanna, Table of n, a(n) for n = 1..200

Cf. A215880, A215881, A215882, A216143.

max = 28; Chw[x_] := (-ProductLog[-x] + ProductLog[x])/(2*x); Shw[x_] := -(ProductLog[-x] + ProductLog[x])/(2*x); se = Series[ Shw[x]/Chw[x], {x, 0, max}]; (CoefficientList[se, x] // DeleteCases[#, 0] &)*Range[1, max, 2]! (* Jean-François Alcover, Jun 24 2013 *)

{a(n)=local(Chw=sum(m=0, n, (2*m+1)^(2*m-1)*x^(2*m)/(2*m)!)+x*O(x^(2*n)),Shw=sum(m=0, n, (2*m+2)^(2*m)*x^(2*m+1)/(2*m+1)!)+x*O(x^(2*n+1))); (2*n+1)!*polcoeff(Shw/Chw, 2*n+1)}
for(n=0,20,print1(a(n),", "))

{a(n)=(2*n+1)!*polcoeff( tanh( sum(m=0, n, (2*m+1)^(2*m)*x^(2*m+1)/(2*m+1)!) +x*O(x^(2*n+1))), 2*n+1)}
for(n=0,20,print1(a(n),", "))

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

A216409 Expansion of e.g.f. (x/cos(x)) * exp(x*tan(x)) (odd powers only).

Original entry on oeis.org

1, 9, 185, 6769, 384849, 31247161, 3421948361, 485057489505, 86270172949025, 18789108183911401, 4913945007420622425, 1518613513007413125073, 547156929866111948071025, 227227144424871839232479769, 107701858026047543489146771049

Offset: 1

Paul D. Hanna, Sep 06 2012

nonn

Name changed Mar 24 2025; prior name was: e.g.f. Series_Reversion( x*Cw(x) ), which is equivalent to formula (2).

			E.g.f.: A(x) = x + 9*x^3/3! + 185*x^5/5! + 6769*x^7/7! + 384849*x^9/9! + ...
Related expansions.
Define Cw(x) and Sw(x) by Cw(x) + i*Sw(x) = LambertW(-i*x)/(-i*x), then
Cw(x) + i*Sw(x) = (cos(x*Cw(x)) + i*sin(x*Cw(x))) * exp(-x*Sw(x)) and
Cw(x) = 1 - 3*x^2/2! + 125*x^4/4! - 16807*x^6/6! + 4782969*x^8/8! -+ ... + (-1)^n*(2*n+1)^(2*n-1)*x^(2*n)/(2*n)! + ...
where A(x*Cw(x)) = x, and
Sw(x) = x - 16*x^3/3! + 1296*x^5/5! - 262144*x^7/7! + 100000000*x^9/9! -+ ... + (-1)^n*(2*n+2)^(2*n)*x^(2*n+1)/(2*n+1)! + ...
where A( arctan(Sw(x)/Cw(x)) ) = x.
Also, Sw(x)/Cw(x) = tan( x*Cw(x) ) and begins
Sw(x)/Cw(x) = x - 7*x^3/3! + 461*x^5/5! - 84827*x^7/7! + 30648697*x^9/9! - 18319873199*x^11/11! +- ... + (-1)^(n-1) * A215890(n)*x^(2*n-1) + ...

Paul D. Hanna, Table of n, a(n) for n = 1..201

Cf. A215890, A138734, A215880, A215881, A215882, A216143.

nmax = 20; Quiet[Rest[CoefficientList[Series[x*E^(Sqrt[x]*Tan[Sqrt[x]])*Sec[Sqrt[x]], {x, 0, nmax}], x] * (2*Range[0, nmax]-1)!]] (* Vaclav Kotesovec, Mar 24 2025 *)

{a(n)=local(Cw=sum(m=0,n,(-1)^m*(2*m+1)^(2*m-1)*x^(2*m)/(2*m)!) +x*O(x^n));n!*polcoeff(serreverse(x*Cw),n)}
for(n=1,20,print1(a(2*n-1),", ")) \\ print only odd-indexed terms

E.g.f. A(x) = Sum_{n>=1} a(n) * x^(2*n-1)/(2*n-1)! satisfies:
(1) Sum_{n>=0} (-1)^n * (2*n+1)^(2*n) * A(x)^(2*n+1)/(2*n+1)! = x.
(2) A( x*Cw(x) ) = x where Cw(x) = Sum_{n>=0} (-1)^n * (2*n+1)^(2*n-1) * x^(2*n)/(2*n)!.
(3) A( arctan(Sw(x)/Cw(x)) ) = x where Sw(x) = Sum_{n>=0} (-1)^n * (2*n+2)^(2*n) * x^(2*n+1)/(2*n+1)!.
(4) A(x) = (x/cos(x)) * exp(x*tan(x)). - Paul D. Hanna, Mar 24 2025
a(n) ~ 2^(4*n - 5/4) * n^(2*n - 3/4) / (Pi^(2*n-1) * exp(2*n - 2*sqrt(2*n) + 1/2)) * (1 - (14 + Pi^2)/(12*sqrt(2*n))). - Vaclav Kotesovec, Mar 24 2025

Name changed and entry revised by Paul D. Hanna, Mar 24 2025

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A215880 Expansion of e.g.f.: sqrt( -LambertW(x)*LambertW(-x)/x^2 ).

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A215881 Expansion of e.g.f.: sqrt( -LambertW(-x) / LambertW(x) ).

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A215890 E.g.f.: Shw(x) / Chw(x), where Chw(x) = Sum_{n>=0} (2*n+1)^(2*n-1) * x^(2*n)/(2*n)! and Shw(x) = Sum_{n>=0} (2*n+2)^(2*n) * x^(2*n+1)/(2*n+1)!, with zero coefficients omitted.

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A216409 Expansion of e.g.f. (x/cos(x)) * exp(x*tan(x)) (odd powers only).

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A215890 E.g.f.: Shw(x) / Chw(x), where Chw(x) = Sum_{n>=0} (2n+1)^(2n-1) * x^(2n)/(2n)! and Shw(x) = Sum_{n>=0} (2n+2)^(2n) * x^(2n+1)/(2n+1)!, with zero coefficients omitted.