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A128457
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Numbers n such that 13^n - 2 is a prime.
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12
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1, 2, 4, 5, 12, 78, 80, 90, 117, 120, 813, 1502, 2306, 2946, 6308, 13320
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,2
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COMMENTS
| 13320 is a term found by Lelio R Paula 11/2006.
Contribution from Farideh Firoozbakht (mymontain(AT)yahoo.com), Dec 07 2009: 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.
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MATHEMATICA
| Do[ f = 13^n - 2; If[ PrimeQ[ f ], Print[ {n, f} ] ], {n, 1, 1000} ]
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CROSSREFS
| Cf. A084714 = smallest prime of the form (2n-1)^k - 2, or 0 if no such number exists. Cf. A128472 = smallest prime of the form (2n-1)^k - 2 for k>(2n-1), or 0 if no such number exists. Cf. A014224, A109080, A090669, A128455, A128458, A128459, A128460, A128461.
Sequence in context: A091071 A050599 A102932 * A139485 A079407 A078652
Adjacent sequences: A128454 A128455 A128456 * A128458 A128459 A128460
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KEYWORD
| hard,more,nonn
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AUTHOR
| Alexander Adamchuk (alex(AT)kolmogorov.com), Mar 14 2007
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EXTENSIONS
| 813 from Stefan Steinerberger (stefan.steinerberger(AT)gmail.com), May 05 2007
a(12) from M. F. Hasler (www.univ-ag.fr/~mhasler), Feb 07 2009
a(13)-a(16) from Farideh Firoozbakht (mymontain(AT)yahoo.com), Dec 07 2009
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