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A178384 A (-1,1) Somos-4 sequence associated with the elliptic curve y^2 + y = x^3 + x.

%I A178384 #37 Sep 15 2024 08:55:43
%S A178384 1,1,1,3,-2,11,-29,21,-305,-764,-3761,-26829,20827,-1044667,7336774,
%T A178384 -34981779,829881529,4656917815,116074261249,2133710863224,
%U A178384 4261714316929,871401830149817,-15861891538169783,387559539627947379,-20207945101587735626,-195471056819748264101
%N A178384 A (-1,1) Somos-4 sequence associated with the elliptic curve y^2 + y = x^3 + x.
%C A178384 Hankel transform of the sequence with g.f. 1/(1-x^2/(1-x^2/(1-3x^2/(1+(2/9)x^2/(1-(33/4)x^2/(1-...)))))) where 1,3,-2/9,33/4,... are the x-coordinates of the multiples of (0,0).
%H A178384 Reinhard Zumkeller, <a href="/A178384/b178384.txt">Table of n, a(n) for n = 0..150</a>
%H A178384 Paul Barry, <a href="https://arxiv.org/abs/1910.00875">Generalized Catalan recurrences, Riordan arrays, elliptic curves, and orthogonal polynomials</a>, arXiv:1910.00875 [math.CO], 2019.
%H A178384 LMFDB, <a href="http://www.lmfdb.org/EllipticCurve/Q/91/a/1">Elliptic Curve 91.a1 (Cremona label 91a1)</a>
%F A178384 a(n) = (-a(n-1)*a(n-3)+a(n-2)^2)/a(n-4), n>3.
%F A178384 From _Michael Somos_, Jan 11 2015: (Start)
%F A178384 a(n) = (-1)^n * a(-2-n) for all n in Z.
%F A178384 0 = a(n)*a(n+4) + a(n+1)*a(n+3) - a(n+2)*a(n+2) for all n in Z.
%F A178384 0 = a(n)*a(n+5) + a(n+1)*a(n+4) - 3*a(n+2)*a(n+3) for all n in Z. (End)
%t A178384 RecurrenceTable[{a[n] == (-a[n-1]*a[n-3] +a[n-2]^2)/a[n-4], a[0] == 1, a[1] == 1, a[2] == 1, a[3] == 3}, a, {n, 0, 30}] (* _G. C. Greubel_, Sep 18 2018 *)
%o A178384 (Haskell)
%o A178384 a178384 n = a178384_list !! n
%o A178384 a178384_list = [1, 1, 1, 3] ++
%o A178384    zipWith div (foldr1 (zipWith subtract) (map b [1..2])) a178384_list
%o A178384    where b i = zipWith (*) (drop i a178384_list) (drop (4-i) a178384_list)
%o A178384 -- _Reinhard Zumkeller_, Sep 15 2014
%o A178384 (Magma) I:=[1,1,1,3]; [n le 4 select I[n] else (-Self(n-1)*Self(n-3)+Self(n-2)^2)/Self(n-4): n in [1..30]]; // _Vincenzo Librandi_, Jun 24 2015
%o A178384 (PARI) m=30; v=concat([1,1,1,3], vector(m-4)); for(n=5, m, v[n] = (-v[n-1]*v[n-3] + v[n-2]^2)/v[n-4]); v \\ _G. C. Greubel_, Sep 18 2018
%o A178384 (PARI) {a(n) = n++; sign(n) * (-1)^(n\2+n+1) * subst( elldivpol( ellinit([0, 0, 1, 1, 0]), abs(n)), x, 0)}; /* _Michael Somos_, Sep 14 2024 */
%Y A178384 Cf. A006720, A247368.
%K A178384 easy,sign
%O A178384 0,4
%A A178384 _Paul Barry_, May 26 2010