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 A210657 a(0)=1; thereafter a(n) = -2*Sum_{k=1..n} binomial(2n,2k)*a(n-k). 10
 1, -2, 22, -602, 30742, -2523002, 303692662, -50402079002, 11030684333782, -3077986048956602, 1066578948824962102, -449342758735568563802, 226182806795367665865622, -134065091768709178087428602, 92423044260377387363207812342, -73323347841467639992211297199002 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS The version without signs has an interpretation as a sum over marked Schröder paths. See the Josuat-Verges and Kim reference. Consider the sequence defined by a(0)=1; thereafter a(n) = c*Sum_{k=1..n} binomial(2n,2k)*a(n-k). For c = -3, -2, -1, 1, 2, 3, 4 this is A210676, A210657, A028296, A094088, A210672, A210674, A249939. Apparently a(n) = 2*(-1)^n*A002114(n). - R. J. Mathar, Mar 01 2015 LINKS Matthieu Josuat-Vergès and Jang Soo Kim, Touchard-Riordan formulas, T-fractions, and Jacobi's triple product identity, arXiv:1101.5608  [math.CO], 2011. Zhi-Hong Sun, On the further properties of U_n, arXiv:1203.5977 [math.NT], 2012. FORMULA O.g.f.: Sum_{n>=0} (2*n)! * (-x)^n / Product_{k=1..n} (1 - k^2*x). - Paul D. Hanna, Sep 17 2012 E.g.f.: 1/(2*cosh(x) - 1) = Sum_{n>=0} a(n)*x^(2*n)/(2*n)!. - Paul D. Hanna, Oct 30 2014 E.g.f.: cos(z/2)/cos(3z/2) = Sum_{n>=0} abs(a(n))*x^(2*n)/(2*n)!. - Olivier Gérard, Feb 12 2014 From Peter Bala, Mar 09 2015: (Start) a(n) = 3^(2*n)*E(2*n,1/3), where E(n,x) is the n-th Euler polynomial. O.g.f.: Sum_{n >= 0} 1/2^n * Sum_{k = 0..n} (-1)^k*binomial(n,k)/(1 - x*(3*k + 1)^2). O.g.f. as a continued fraction: 1/(1 + (3^2 - 1^2)*x/(4 + 12^2*x/(4 + (18^2 - 2^2)*x/(4 + 24^2*x/(4 + (30^2 - 2^2)*x/(4 + 36^2*x/(4 + ... ))))))) = 1 - 2*x + 22*x^2 - 602*x^3 + 30742*x^4 - .... See Josuat-Vergès and Kim, p. 23. The expansion of exp( Sum_{n >= 1} a(n)*x^n/n ) appears to have integer coefficients. See A255882. (End) a(n) = 2*36^n*(zeta(-n*2,1/6)-zeta(-n*2,2/3)), where zeta(a,z) is the generalized Riemann zeta function. - Peter Luschny, Mar 11 2015 a(n) ~ 2 * (-1)^n * (2*n)! * 3^(2*n+1/2) / Pi^(2*n+1). - Vaclav Kotesovec, Mar 14 2015 MAPLE A210657:=proc(n) option remember;    if n=0 then 1    else -2*add(binomial(2*n, 2*k)*procname(n-k), k=1..floor(n)); fi; end; [seq(f(n), n=0..20)]; # Second program: a := (n) -> 2*36^n*(Zeta(0, -n*2, 1/6)-Zeta(0, -n*2, 2/3)): seq(a(n), n=0..15); # Peter Luschny, Mar 11 2015 MATHEMATICA nmax=20; Table[(CoefficientList[Series[1/(2*Cosh[x]-1), {x, 0, 2*nmax}], x] * Range[0, 2*nmax]!)[[2*n+1]], {n, 0, nmax}] (* Vaclav Kotesovec, Mar 14 2015 *) Table[9^n EulerE[2 n, 1/3], {n, 0, 20}] (* Vladimir Reshetnikov, Jun 05 2016 *) PROG (PARI) a(n)=polcoeff(sum(m=0, n, (2*m)!*(-x)^m/prod(k=1, m, 1-k^2*x +x*O(x^n)) ), n) for(n=0, 20, print1(a(n), ", ")) \\ Paul D. Hanna, Sep 17 2012 CROSSREFS Cf. A002114, A028296, A094088, A210657, A210672, A210674, A210676, A249939, A255882. Sequence in context: A248798 A120419 A217912 * A177042 A318639 A220732 Adjacent sequences:  A210654 A210655 A210656 * A210658 A210659 A210660 KEYWORD sign AUTHOR N. J. A. Sloane, Mar 28 2012 STATUS approved

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Last modified February 21 15:11 EST 2019. Contains 320374 sequences. (Running on oeis4.)