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 A178081 A (1,1) Somos-4 sequence. 2
 0, 1, 1, -1, 2, 3, 1, -11, -16, 35, -129, -299, -386, 3977, 8063, -42489, 269344, 1000009, 3727745, -47166649, -123526014, 1764203419, -18228952703, -113727892147, -1065812586544, 18344075481339, 52130069331199, -2470319425874195 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,5 COMMENTS Hankel transform of A178080 is a(n+2). From Paul Barry, May 31 2010: (Start) The sequence 1,1,-1,2,3,... is associated to the elliptic curve E:y^2+xy-y=x^3+x^2-x (see PARI code below). This is also (-1)^C(n,2) times the Hankel transform of the sequence whose g.f. is 1/(1-x^2/(1-x^2/(1+2x^2/(1-(3/4)x^2/(1+(2/9)x^2/(1-...)))))) where 1, -2, 3/4, -2/9, 33, ... are the x-coordinates of the multiples of z=(0,0) on the elliptic curve E:y^2+xy-y=x^3+x^2-x. (End) This is a strong elliptic divisibility sequence t_n as given in [Kimberling, p. 16] where x = 1, y = -1, z = 2. - Michael Somos, Aug 06 2014 This is associated with elliptic curve y^2 + xy - y = x^3 + x^2 - 2x (Cremona label 79a1) and multiples of the point (0, 0). - Michael Somos, Feb 15 2023 LINKS G. C. Greubel, Table of n, a(n) for n = 0..215 C. Kimberling, Strong divisibility sequences and some conjectures, Fib. Quart., 17 (1979), 13-17. LMFDB, Elliptic Curve 79.a1 (Cremona label 79a1) FORMULA a(n) = (a(n-1)*a(n-3) + a(n-2)^2)/a(n-4), n>=4. a(-n) = -a(n) for all n in Z. - Michael Somos, Aug 06 2014 EXAMPLE G.f. = x + x^2 - x^3 + 2*x^4 + 3*x^5 + x^6 - 11*x^7 - 16*x^8 + 35*x^9 + ... MATHEMATICA Join[{0}, RecurrenceTable[{a[1]==1, a[2]==1, a[3]==-1, a[4]==2, a[n]==(a[n-1]a[n-3]+a[n-2]^2)/a[n-4]}, a, {n, 30}]] (* Harvey P. Dale, Sep 07 2016 *) PROG (PARI) a(n)=local(E, z); E=ellinit([1, 1, -1, -1, 0]); z=ellpointtoz(E, [0, 0]); round(ellsigma(E, n*z)/ellsigma(E, z)^(n^2)) /* Paul Barry, May 31 2010 */ (Magma) I:=[0, 1, 1, -1, 2]; [n le 5 select I[n] else (Self(n-1)*Self(n-3)+Self(n-2)^2)/Self(n-4): n in [1..50]]; // Vincenzo Librandi, Aug 06 2014 (GAP) a:=[1, 1, -1, 2];; for n in [5..30] do a[n]:=(a[n-1]*a[n-3]+a[n-2]^2)/a[n-4]; od; a:=Concatenation([0], a); # Muniru A Asiru, Sep 23 2018 (SageMath) @CachedFunction def a(n): # a = A178081 if (n<5): return (0, 1, 1, -1, 2)[n] else: return (a(n-1)*a(n-3) + a(n-2)^2)/a(n-4) [a(n) for n in range(41)] # G. C. Greubel, Feb 16 2023 CROSSREFS Cf. A174017. Sequence in context: A343773 A100223 A174017 * A368272 A129969 A104379 Adjacent sequences: A178078 A178079 A178080 * A178082 A178083 A178084 KEYWORD easy,sign AUTHOR Paul Barry, May 19 2010 EXTENSIONS a(0)=0 prepended by Michael Somos, Aug 06 2014 a(1)=1 added also by Michael Somos, Feb 15 2023 STATUS approved

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Last modified July 13 14:24 EDT 2024. Contains 374284 sequences. (Running on oeis4.)