A000816 E.g.f.: Sum_{n >= 0} a(n) * x^(2*n) / (2*n)! = sin(x)^2 / cos(2*x).

0, 2, 40, 1952, 177280, 25866752, 5535262720, 1633165156352, 635421069967360, 315212388819402752, 194181169538675507200, 145435130631317935357952, 130145345400688287667978240, 137139396592145493713802493952

Offset

0,2

Links

R. J. Mathar, Table of n, a(n) for n = 0..200

Formula

(1/2) * A002436(n), n > 0. - Ralf Stephan, Mar 09 2004

a(n) = 2^(2*n - 1) * A000364(n) except at n=0.

E.g.f.: sin(x)^2/cos(2x) = 1/Q(0) - 1/2; Q(k) = 1 + 1/(1-2*(x^2)/(2*(x^2)-(k+1)*(2k+1)/Q(k+1))); (continued fraction). - Sergei N. Gladkovskii, Nov 18 2011

a(n) = A000819(n) unless n=0.

G.f.: (1/(G(0))-1)/2 where G(k) = 1 - 4*x*(k+1)^2/G(k+1); (continued fraction). - Sergei N. Gladkovskii, Jan 12 2013

G.f.: T(0)/2 - 1/2, where T(k) = 1 - 4*x*(k+1)^2/( 4*x*(k+1)^2 - 1/T(k+1) ); (continued fraction). - Sergei N. Gladkovskii, Oct 25 2013

E.g.f.: sin(x)^2/cos(2*x) = x^2/(1-2*x^2)*T(0), where T(k) = 1 - x^2*(2*k+1)*(2*k+2)/( x^2*(2*k+1)*(2*k+2) + ((k+1)*(2*k+1) - 2*x^2)*((k+2)*(2*k+3) - 2*x^2)/T(k+1) ); (continued fraction). - Sergei N. Gladkovskii, Oct 25 2013

From Artur Jasinski, Mar 21 2022: (Start)

For n > 0:

a(n) = Pi^(2*n-1)*(-Psi(2*n,1/4) - (4^n)*(2^(2*n+1)-1)*Gamma(2*n+1)*Zeta(2*n+1)).

a(n) = (-1)^(n+1)*2^(2*n)*i*Li_(2*n,i) where i=sqrt(-1) and Li is polylogarithm function.

a(n) = (-64)^n*(zeta(-2*n,1/4)-zeta(-2*n,3/4)) where zeta is Hurwitz zeta function.

a(n) = (-16)^n*lerchphi(-1,-2*n,1/2). (End)

Mathematica

Union[ Range[0, 26]! CoefficientList[ Series[ Sin[x]^2/Cos[ 2x], {x, 0, 26}], x]] (* Robert G. Wilson v, Apr 16 2011 *)
Table[(-1)^(n + 1) 2^(2 n) I PolyLog[-2 n, I], {n, 1, 13}] (* Artur Jasinski, Mar 21 2022 *)
With[{nn=30}, Take[CoefficientList[Series[Sin[x]^2/Cos[2x], {x, 0, nn}], x] Range[0, nn]!, {1, -1, 2}]] (* Harvey P. Dale, Oct 18 2024 *)

Prog

(PARI) {a(n) = local(m); if( n<0, 0, m = 2*n; m! * polcoeff( 1 / (2 - 1 / cos(x + x * O(x^m))^2) - 1, m))} /* Michael Somos, Apr 16 2011 */
(Sage)
@CachedFunction
def sp(n, x) :
    if n == 0 : return 1
    return -add(2^(n-k)*sp(k, 1/2)*binomial(n, k) for k in range(n)[::2])
def A000816(n) : return 0 if n == 0 else abs(sp(2*n, x)/2)
[A000816(n) for n in (0..13)]   # Peter Luschny, Jul 30 2012

Crossrefs

Cf. A000364, A000819, A000822, A000828, A003707, A009125, A009569.

Sequence in context: A261732 A292418 A163826 * A000819 A060079 A052502

Adjacent sequences: A000813 A000814 A000815 * A000817 A000818 A000819

Keyword

nonn

Author

N. J. A. Sloane

Status

approved