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A074477
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Largest prime factor of 3^n - 1.
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12
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2, 2, 13, 5, 11, 13, 1093, 41, 757, 61, 3851, 73, 797161, 1093, 4561, 193, 34511, 757, 363889, 1181, 368089, 3851, 1001523179, 6481, 391151, 797161, 8209, 16493, 20381027, 4561, 4404047, 21523361, 2413941289, 34511, 2664097031, 530713
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OFFSET
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1,1
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LINKS
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FORMULA
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EXAMPLE
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3^7 - 1 = 2186 = 2*1093, so a(7) = 1093.
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MAPLE
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# alternative:
a:= n-> max(seq(i[1], i=ifactors(3^n-1)[2])):
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MATHEMATICA
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Table[FactorInteger[3^n - 1] [[-1, 1]], {n, 40}] (* Vincenzo Librandi, Aug 23 2013 *)
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PROG
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(PARI) for(n=1, 40, v=factor(3^n-1); print1(v[matsize(v)[1], 1], ", "))
(Magma) [Maximum(PrimeDivisors(3^n-1)): n in [1..40]]; // Vincenzo Librandi, Aug 23 2013
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CROSSREFS
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Cf. A074476 (largest prime factor of 3^n + 1), A005420 (largest prime factor of 2^n - 1), A074479 (largest prime factor of 5^n - 1).
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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