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A217384
Numbers k such that 9^k + 4 is prime.
4
0, 1, 3, 5, 11, 15, 21, 87, 99, 281, 497, 2919, 6849, 7365, 8483, 49317, 58611
OFFSET
1,3
COMMENTS
Contribution from Bruno Berselli, Oct 04 2012: (Start)
Contains exactly the halved even terms of A058958.
Naturally, apart from the first term, these numbers are odd since (10-1)^(2n)+4 is divisible by 5. (End)
a(18) > 10^5. - Tyler NeSmith, May 05 2021
MATHEMATICA
Select[Range[0, 3000], PrimeQ[9^# + 4] &]
PROG
(PARI) is(n)=ispseudoprime(9^n+4) \\ Charles R Greathouse IV, Feb 20 2017
CROSSREFS
Sequence in context: A105772 A244520 A034169 * A335279 A136977 A003529
KEYWORD
nonn,hard,more,changed
AUTHOR
Vincenzo Librandi, Oct 04 2012
EXTENSIONS
a(13)-a(14) from Bruno Berselli, Oct 04 2012
a(15)-a(17) from Tyler NeSmith, May 05 2021
STATUS
approved