A217384 Numbers k such that 9^k + 4 is prime.

0, 1, 3, 5, 11, 15, 21, 87, 99, 281, 497, 2919, 6849, 7365, 8483, 49317, 58611

Offset: 1

Author

Vincenzo Librandi, Oct 04 2012

Keywords

• nonn
• hard
• more

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

Crossrefs

Cf. A058958, A090649.

Programs

Mathematica

Select[Range[0, 3000], PrimeQ[9^# + 4] &]

PARI

is(n)=ispseudoprime(9^n+4) \\ Charles R Greathouse IV, Feb 20 2017

Extensions

a(13)-a(14) from Bruno Berselli, Oct 04 2012

a(15)-a(17) from Tyler NeSmith, May 05 2021