A097496 - OEIS

%I #20 Jun 26 2017 23:04:28

%S 1,1,2,5,37,274,6161,165713,9434290,1013908933,142844426789,

%T 57760865728994,23510036246274433,29636604976524724225,

%U 42939094881045167060258,139127528853472250968193381,802780326236562434930932841989,6388127622103180665094420305200242

%N Subsequence of terms of odd index of the Somos-5 sequence.

%C The sequence corresponds to the sequence of points Q+nP on the curve y^2=4*x^3-(121/12)*x+845/216, where Q=(-7/12,-3) and P=(17/12,-1).

%C Another sequence of points is (2*n - 1)*P on the curve "102a1": y^2 + x * y = x^3 + x^2 - 2 * x where P = [-1, -1]. - _Michael Somos_, Jul 15 2011

%H A. N. W. Hone, <a href="http://dx.doi.org/10.1112/S0024609304004163">Elliptic curves and quadratic recurrence sequences</a>, Bull. Lond. Math. Soc. 37 (2005) 161-171.

%F a(0)=a(1)=1, a(2)=2, a(3)=5; a(n) = (a(n-1)*a(n-3)+8*a(n-2)^2)/a(n-4).

%F Exact formula: a(n)=D*E^n*sigma(f+n*k)/sigma(k)^(n^2) where sigma is the Weierstrass sigma function associated to the elliptic curve y^2=4*x^3-(121/12)*x+845/216, D=1/sigma(f)=-0.363554228-0.803200610*I, E=sigma(k)*sigma(f)/sigma(f+k)=0.644801269+0.734118205*I, f=integral{infinity}^{-7/12} dx/y = -0.509286773+0.973928783*I, k=integral{17/12}^{infinity} dx/y = 1.018573545 all to 9 d.p.

%F a(n) = a(1 - n), a(n) = (-8 * a(n-1) * a(n-4) + 57 * a(n-2) * a(n-3)) / a(n-5) for all n in Z. - _Michael Somos_, Jul 15 2011

%p a[0]:=1; a[1]:=1;a[2]:=2;a[3]:=5; for n from 1 to 20 do a[n+3]:=(a[n+2]*a[n]+8*a[n+1]^2)/a[n-1] od;

%o (PARI) {a(n) = sqrtint( denominator( ellpow( ellinit( [1, 1, 0, -2, 0], 1), [-1, -1], 2*n - 1)[1]))}; /* _Michael Somos_, Jul 15 2011 */

%Y Cf. A006721, A006720.

%K nonn

%O 0,3

%A _Andrew Hone_, Aug 24 2004