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A178624
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A (1,3) Somos-4 sequence associated to the elliptic curve E: y^2 + 2*x*y - y = x^3 - x.
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3
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1, 1, -3, 11, 38, 249, -2357, 8767, 496035, -3769372, -299154043, -12064147359, 632926474117, -65604679199921, -6662962874355342, -720710377683595651, 285131375126739646739, 5206174703484724719135
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OFFSET
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1,3
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COMMENTS
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a(n) is (-1)^C(n,2) times the Hankel transform of the sequence with g.f. 1/(1-x^2/(1-3x^2/(1+(11/9)x^2/(1-(114/121)x^2/(1+(2739/1444)x^2/(1-... where 3,-11/9,141/121,-2739/1444... are the x-coordinates of the multiples of z=(0,0) on E:y^2+2xy-y=x^3-x.
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LINKS
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FORMULA
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a(n) = (a(n-1)*a(n-3) + 3*a(n-2)^2)/a(n-4), n>3.
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EXAMPLE
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G.f. = x + x^2 - 3*x^3 + 11*x^4 + 38*x^5 + 249*x^6 + ... - Michael Somos, Sep 17 2018
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MATHEMATICA
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RecurrenceTable[{a[n] == (a[n-1]*a[n-3] +3*a[n-2]^2)/a[n-4], a[0] == 1, a[1] == 1, a[2] == -3, a[3] == 11}, a, {n, 0, 30}] (* G. C. Greubel, Sep 16 2018 *)
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PROG
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(PARI) a(n)=local(E, z); E=ellinit([2, 0, -1, -1, 0]); z=ellpointtoz(E, [0, 0]); round(ellsigma(E, n*z)/ellsigma(E, z)^(n^2))
(PARI) m=30; v=concat([1, 1, -3, 11], vector(m-4)); for(n=5, m, v[n] = ( v[n-1]*v[n-3] + 3*v[n-2]^2)/v[n-4]); v \\ G. C. Greubel, Sep 16 2018
(Magma) I:=[1, 1, -3, 11]; [n le 4 select I[n] else (Self(n-1)*Self(n-3) + 3*Self(n-2)^2)/Self(n-4): n in [1..30]]; // G. C. Greubel, Sep 16 2018
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CROSSREFS
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KEYWORD
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easy,sign
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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