A204322 Numbers k such that 4*5^k + 1 is prime.

0, 2, 6, 18, 50, 290, 2582, 20462, 23870, 26342, 31938, 38122, 65034, 70130, 245538, 851878, 1052422, 1496566, 5380542, 8431178

Offset: 1

Author

José María Grau Ribas, Jan 14 2012

Keywords

• nonn
• more

Comments

All terms correspond to verified primes, that is, not merely probable primes. - Robert Price, Jan 16 2015

a(16) > 3*10^5. - Robert Price, Jan 16 2015

Programs

Mathematica

Select[Range[0,2000], PrimeQ[4*5^#+1]&]

PARI

is(n)=ispseudoprime(4*5^n+1) \\ Charles R Greathouse IV, Jun 13 2017

Extensions

a(8)-a(15) from Robert Price, Jan 16 2015

a(16)-a(19) from Ryan Propper, Dec 20 2023

a(20) from Ryan Propper, Jan 04 2024