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A025164
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a(n) = a(n-2) + (2n-1)a(n-1); a(0)=1, a(1)=1.
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5
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1, 1, 4, 21, 151, 1380, 15331, 200683, 3025576, 51635475, 984099601, 20717727096, 477491822809, 11958013297321, 323343850850476, 9388929687961125, 291380164177645351, 9624934347550257708, 337164082328436665131, 12484695980499706867555, 487240307321817004499776
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OFFSET
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0,3
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COMMENTS
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Numerators of convergents to coth(1) = 1.313035... = A073747.
Numerator of continued fraction given by C(n) = [ 1; 3, 5, 7, ..., (2n-1)]. - Amarnath Murthy, May 02 2001
Equals eigensequence of an infinite lower triangular matrix with (1, 3, 5, ...) in the main diagonal, (1, 1, 1, ...) in the sum diagonal, and the rest zeros. - Gary W. Adamson, Apr 17 2009
We can use the defining recurrence to extend the sequence to negative indices to give a(-n) = A036244(n-1). - Peter Bala, Sep 11 2014
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LINKS
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FORMULA
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E.g.f.: cosh((1-2*x)^(1/2)-1)/(1-2*x)^(1/2). - Vladeta Jovovic, Jan 30 2004
a(n) = round((exp(1)+exp(-1))*(BesselK(n-3/2, 1)+(2*n-1)*BesselK(n-1/2, 1))/sqrt(2*Pi) ). - Mark van Hoeij, Jul 02 2010
a(n) = A001147(n)*hypergeometric([-n/2+1/2, -n/2], [1/2, -n, 1/2-n], 1) for n >= 1. - Peter Luschny, Sep 11 2014
a(n) = Sum_{k = 0..floor(n/2)} binomial(n - k, k)*( Product_{j = 1 .. n - 2*k} (2*k + 2*j - 1) ) = Sum_{k = 0..floor((n+1)/2)} 2^(2*k - n - 1)*(2*n + 2 - 2*k)!/( (n + 1 - 2*k)!*(2*k)! ). - Peter Bala, Sep 11 2014
a(n) = -i*( BesselK(1/2, 1)*BesselI(n+1/2, -1) - BesselI(-1/2, -1)*BesselK(n+1/2, 1)) for n>=0 (a(0)=1, a(1) = 1). - G. C. Greubel, Apr 21 2015
a(n) = A036244(-1-n) for all n in Z.
0 = a(n)*(-a(n+2)) + a(n+1)*(+a(n+1) + 2*a(n+2) - a(n+3)) + a(n+2)*(+a(n+2)) if n >= 0. - Michael Somos, Jan 10 2017
Given e.g.f. A(x), then 0 = A(x) + 3*A'(x) + (2*x-1)*A''(x). - Michael Somos, Jan 10 2017
Given g.f. A(x), then 0 = 1 + (x^2+x-1)*A(x) + 2*x^2*A'(x). - Michael Somos, Jan 10 2017
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EXAMPLE
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G.f. = 1 + x + 4*x^2 + 21*x^3 + 151*x^4 + 1380*x^5 + 15331*x^6 + ...
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MAPLE
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a:= proc(n) option remember;
`if`(n<2, 1, a(n-2) +(2*n-1)*a(n-1))
end:
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MATHEMATICA
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a[ n_ ] := a[n] =a[n-2]+(-1+2 n) a[n-1]; a[0] := 1; a[1] := 1;
RecurrenceTable[{a[0]==a[1]==1, a[n]==a[n-2]+(2n-1)a[n-1]}, a, {n, 20}] (* Harvey P. Dale, Mar 25 2012 *)
a[ n_] := Round[ (Exp[1] + Exp[-1]) (BesselK[n - 3/2, 1] + (2 n - 1) BesselK[n - 1/2, 1]) / Sqrt[2 Pi]]; (* Michael Somos, Aug 26 2015 (n>=0) *)
a[ n_] := Module[{ y = Sqrt[1 - 2 x]}, n! SeriesCoefficient[ Cosh[y - 1] / y, {x, 0, n}]]; (* Michael Somos, Aug 26 2015 (n>=0) *)
a[ n_] := (BesselK[ 1/2, 1] BesselI[ n + 1/2, -1] - BesselI[ -1/2, -1] BesselK[ n + 1/2, 1]) / I // FunctionExpand // Simplify; (* Michael Somos, Aug 26 2015 *)
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PROG
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(Sage)
if n == 0: return 1
return sloane.A001147(n)*hypergeometric([-n/2+1/2, -n/2], [1/2, -n, 1/2-n], 1)
(Magma) [n le 2 select 1 else (2*n-3)*Self(n-1)+Self(n-2): n in [1..20]] // Vincenzo Librandi, Apr 22 2015
(PARI) a(n)={if(n<2, 1, a(n-2)+(2*n-1)*a(n-1))} \\ Edward Jiang, Sep 11 2014
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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More terms from Larry Reeves (larryr(AT)acm.org), May 15 2001
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STATUS
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approved
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