A308449 Numbers k such that 25*10^(2*k) + 5*10^k + 1 are prime.

0, 1, 2, 14, 174, 448, 16562

Offset: 1

Author

Seiichi Manyama, Aug 15 2019

Keywords

• nonn
• more
• base

Examples

			          31 is prime ==> a(1) = 0.
        2551 is prime ==> a(2) = 1.
      250501 is prime ==> a(3) = 2.
    25005001 = 7 * 79 * 103 * 439.
  2500050001 = 19 * 2269 * 57991.
250000500001 = 7 * 103561 * 344863.
		

Crossrefs

Cf. A309738.

Programs

PARI

for(k=0, 1e3, if(ispseudoprime(25*100^k+5*10^k+1), print1(k", ")))

Extensions

a(7) from Michael S. Branicky, Apr 17 2023