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

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

Offset

1,3

Links

Table of n, a(n) for n=1..7.

Example

          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.

Prog

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

Crossrefs

Cf. A309738.

Sequence in context: A235369 A351274 A228476 * A233224 A167014 A370909

Adjacent sequences: A308446 A308447 A308448 * A308450 A308451 A308452

Keyword

nonn,more,base

Author

Seiichi Manyama, Aug 15 2019

Extensions

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

Status

approved