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A171422
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A (4,-5) Somos-4 sequence.
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3
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1, -1, -9, -41, -241, -271, 27329, 651521, 18425809, 399122991, -24055184809, -3943757411849, -498726356978849, -74609317288333151, 2771780972000481601, 8459493968101503667649, 5139175203350482789079649
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OFFSET
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0,3
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COMMENTS
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LINKS
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FORMULA
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a(n) = (4*a(n-1)*a(n-3) - 5*a(n-2)^2)/a(n-4), n>=3.
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MATHEMATICA
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RecurrenceTable[{a[n] == (4*a[n-1]*a[n-3] -5*a[n-2]^2)/a[n-4], a[0] == 1, a[1] == -1, a[2] == -9, a[3] == -41}, a, {n, 0, 30}] (* G. C. Greubel, Sep 18 2018 *)
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PROG
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(PARI) m=30; v=concat([1, -1, -9, -41], vector(m-4)); for(n=5, m, v[n] = ( 4*v[n-1]*v[n-3] -5*v[n-2]^2)/v[n-4]); v \\ G. C. Greubel, Sep 18 2018
(Magma) I:=[1, -1, -9, -41]; [n le 4 select I[n] else (4*Self(n-1)*Self(n-3) -5*Self(n-2)^2)/Self(n-4): n in [1..30]]; // G. C. Greubel, Sep 18 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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STATUS
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approved
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