A059613 Numbers k such that 5^k - 4 is prime.

5, 7, 15, 47, 81, 115, 267, 285, 7641, 19089, 25831, 32115, 59811, 70155, 178715

Offset: 1

Author

Andrey V. Kulsha, Feb 07 2001

Keywords

• nonn
• hard
• more

Comments

a(15) > 10^5. - Robert Price, Feb 03 2014

a(16) > 2*10^5. - Tyler NeSmith, Jul 25 2021

Examples

			81 is present because 5^81 - 4 is prime.
		

Crossrefs

Cf. A164785.

Programs

Mathematica

Select[Range[10000], PrimeQ[5^# - 4] &] (* Vincenzo Librandi, Oct 03 2012 *)

PARI

is(n)=ispseudoprime(5^n-4) \\ Charles R Greathouse IV, Feb 20 2017

Extensions

a(10)-a(14) from Robert Price, Feb 03 2014

a(15) from Tyler NeSmith, Jul 25 2021