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A128457
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Numbers k such that 13^k - 2 is a prime.
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14
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1, 2, 4, 5, 12, 78, 80, 90, 117, 120, 813, 1502, 2306, 2946, 6308, 13320, 26369, 31868, 44265, 81008
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OFFSET
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1,2
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COMMENTS
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13320 is a term found by Lelio R Paula 11/2006.
Numbers corresponding to a(13)..a(16) are probable primes. If n is of the form 4k+3 then 13^n-2 is composite, because 13^n-2 == (3^4)^k*3^3 - 2 == 25 == 0 (mod 5). So there is no term of the form 4k+3. - Farideh Firoozbakht, Dec 07 2009
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LINKS
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MATHEMATICA
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Do[ f = 13^n - 2; If[ PrimeQ[ f ], Print[ {n, f} ] ], {n, 1, 1000} ]
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CROSSREFS
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Cf. A084714 (smallest prime of the form (2n-1)^k - 2).
Cf. A128472 (smallest prime of the form (2n-1)^k - 2 for k > (2n-1)).
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KEYWORD
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nonn,hard,more
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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