A228076 Numbers k such that (19^k - 4^k)/15 is prime.

2, 7, 11, 31, 47, 139, 263, 769, 20551, 21491, 112247

Offset: 1

Author

Robert Price, Aug 09 2013

Keywords

• nonn
• hard
• more

Comments

All terms are prime.

a(12) > 2*10^5.

Crossrefs

Cf. A224691, A128354, A128032.

Programs

Mathematica

Select[Prime[Range[1, 100000]], PrimeQ[(19^# - 4^#)/15]&]

PARI

is(n)=ispseudoprime((19^n-4^n)/15) \\ Charles R Greathouse IV, Jun 13 2017

Extensions

a(11) from Robert Price, Mar 06 2021