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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
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1,2
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COMMENTS
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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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LINKS
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Table of n, a(n) for n=1..4.
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MATHEMATICA
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Select[Range[500], PrimeQ[7^Prime[#]-2]&] (* From Harvey P. Dale, May 02 2011 *)
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PROG
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(PARI) forprime(p=2, 1e4, if(ispseudoprime(7^p-2), print1(x", ")))
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CROSSREFS
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Cf. A147782 (primes p such that 7^p - 2 is prime).
Sequence in context: A018242 A006248 A119571 * A118197 A092658 A216571
Adjacent sequences: A089917 A089918 A089919 * A089921 A089922 A089923
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KEYWORD
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nonn
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AUTHOR
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Cino Hilliard (hillcino368(AT)gmail.com), Jan 11 2004
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EXTENSIONS
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Definition clarified by Harvey P. Dale, May 02 2011.
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STATUS
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approved
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