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 A217384 Numbers n such that 9^n + 4 is prime. 4
 0, 1, 3, 5, 11, 15, 21, 87, 99, 281, 497, 2919, 6849, 7365 (list; graph; refs; listen; history; text; internal format)
 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) LINKS MATHEMATICA Select[Range[0, 3000], PrimeQ[9^# + 4] &] PROG (PARI) is(n)=ispseudoprime(9^n+4) \\ Charles R Greathouse IV, Feb 20 2017 CROSSREFS Cf. A058958, A090649. Sequence in context: A105772 A244520 A034169 * A136977 A003529 A018667 Adjacent sequences:  A217381 A217382 A217383 * A217385 A217386 A217387 KEYWORD nonn,hard,more AUTHOR Vincenzo Librandi, Oct 04 2012 EXTENSIONS a(13)-a(14) from Bruno Berselli, Oct 04 2012 STATUS approved

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Last modified December 10 14:27 EST 2019. Contains 329896 sequences. (Running on oeis4.)