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 A053988 Denominators of successive convergents to tan(1/2) using continued fraction 1/(2-1/(6-1/(10-1/(14-1/(18-1/(22-1/(26-1/30-...))))))). 5
 2, 11, 108, 1501, 26910, 590519, 15326584, 459207001, 15597711450, 592253828099, 24859063068708, 1142924647332469, 57121373303554742, 3083411233744623599, 178780730183884614000, 11081321860167101444401, 731188462040844810716466, 51172111020998969648708219 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS G. C. Greubel, Table of n, a(n) for n = 1..360 FORMULA a(n) = Sum_{k=0..floor(n/2)} (-1)^k*(2*n-2*k)!/((n-2*k)!*(2*k)!) - Benoit Cloitre, Jan 03 2006 From G. C. Greubel, May 13 2020: (Start) E.g.f.: cos((1 - sqrt(1-4*x))/2)/sqrt(1-4*x) - 1. a(n) = 2*(2*n-1)*a(n-1) - a(n-2). a(n) = ((-i)^n/2)*(y(n, 2*i) + (-1)^n*y(n, -2*i)), where y(n, x) are the Bessel Polynomials. (End) a(n) ~ cos(1/2) * 2^(2*n + 1/2) * n^n / exp(n). - Vaclav Kotesovec, May 14 2020 MAPLE a:= n -> add((-1)^k*(2*n-2*k)!/((n-2*k)!*(2*k)!), k = 0..floor(n/2)); seq(a(n), n = 1..20); # G. C. Greubel, May 13 2020 MATHEMATICA Table[Sum[(-1)^k*(2*n-2*k)!/((n-2*k)!*(2*k)!), {k, 0, Floor[n/2]}], {n, 20}] (* G. C. Greubel, May 13 2020 *) PROG (PARI) a(n)=sum(k=0, floor(n/2), (-1)^k*(2*n-2*k)!/(n-2*k)!/(2*k)!) \\ Benoit Cloitre, Jan 03 2006 (MAGMA) [(&+[ (-1)^k*Factorial(2*n-2*k)/(Factorial(n-2*k)*Factorial(2*k)): k in [0..Floor(n/2)]] ): n in [1..20]]; // G. C. Greubel, May 13 2020 (Sage) [sum((-1)^k*factorial(2*n-2*k)/(factorial(n-2*k)*factorial(2*k)) for k in (0..floor(n/2))) for n in (1..20)] # G. C. Greubel, May 13 2020 CROSSREFS Cf. A001497, A053987 (numerators), A161011 (tan(1/2)). Sequence in context: A198001 A207155 A292566 * A141314 A099933 A098437 Adjacent sequences:  A053985 A053986 A053987 * A053989 A053990 A053991 KEYWORD easy,frac,nonn AUTHOR Vladeta Jovovic, Apr 03 2000 EXTENSIONS More terms from G. C. Greubel, May 13 2020 STATUS approved

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Last modified October 29 18:53 EDT 2020. Contains 338067 sequences. (Running on oeis4.)