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A089920
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Indices of primes p such that 7^p - 2 is prime.
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0
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OFFSET
| 1,2
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COMMENTS
| Except for p=2, 2, 5^p - 2 cannot be prime. This immediately follows from the fact that a number N = (3k+2)^p - 2 cannot be prime for p > 2 because N = 3H + 2^p - 2 = 3H + 2(2^(p-1)-1) is divisible by 3.
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MATHEMATICA
| Select[Range[500], PrimeQ[7^Prime[#]-2]&] (* From Harvey P. Dale, May 02 2011 *)
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PROG
| (PARI) forprime(p=2, 1e4, if(ispseudoprime(7^p-2), print1(x", ")))
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CROSSREFS
| Cf. A147782 (primes p such that 7^p - 2 is prime).
Sequence in context: A018242 A006248 A119571 * A118197 A092658 A113314
Adjacent sequences: A089917 A089918 A089919 * A089921 A089922 A089923
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KEYWORD
| nonn
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AUTHOR
| Cino Hilliard (hillcino368(AT)gmail.com), Jan 11 2004
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EXTENSIONS
| Definition clarified by Harvey P. Dale, May 02 2011.
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