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A053988
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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-...))))))).
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5
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2, 11, 108, 1501, 26910, 590519, 15326584, 459207001, 15597711450, 592253828099, 24859063068708, 1142924647332469, 57121373303554742, 3083411233744623599, 178780730183884614000, 11081321860167101444401, 731188462040844810716466, 51172111020998969648708219
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OFFSET
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1,1
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LINKS
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FORMULA
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a(n) = Sum_{k=0..floor(n/2)} (-1)^k*(2*n-2*k)!/((n-2*k)!*(2*k)!) - Benoit Cloitre, Jan 03 2006
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)
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MAPLE
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a:= n -> add((-1)^k*(2*n-2*k)!/((n-2*k)!*(2*k)!), k = 0..floor(n/2));
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MATHEMATICA
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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 *)
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PROG
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(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
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CROSSREFS
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KEYWORD
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easy,frac,nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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