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A172995
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Numbers k such that (3^k + 7)/2 is prime.
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2
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OFFSET
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1,2
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COMMENTS
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The primes are in A172996.
Naturally these numbers are odd since 3^(2*k) + 7 = (8+1)^k + 7 == 0 (mod 8). - Bruno Berselli, Oct 08 2012
For n <= 9, (3^a(n) + 7)/2 has been proved prime using PARI's ECPP; a(10) corresponds to a BPSW PRP. a(11) > 10^5. - Lucas A. Brown, Feb 10 2021
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LINKS
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Table of n, a(n) for n=1..10.
Lucas A. Brown, A172995.py
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MATHEMATICA
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Select[Range[5000], PrimeQ[(3^# + 7) / 2 ]&] (* Vincenzo Librandi, Oct 05 2012 *)
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PROG
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(PARI) is(n)=ispseudoprime((3^n+7)/2) \\ Charles R Greathouse IV, Jun 13 2017
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CROSSREFS
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Sequence in context: A075611 A191485 A119572 * A325476 A234536 A103737
Adjacent sequences: A172992 A172993 A172994 * A172996 A172997 A172998
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KEYWORD
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nonn,hard,more
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AUTHOR
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Vincenzo Librandi, Feb 07 2010
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EXTENSIONS
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a(7)-(9) from Vincenzo Librandi, Oct 05 2012
a(10) from Lucas A. Brown, Feb 10 2021
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STATUS
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approved
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