A309740 Numbers k such that 5*10^(2*k) + 5*10^k + 1 is prime.

0, 3, 5, 301, 13817, 15259

Offset: 1

Author

Seiichi Manyama, Aug 15 2019

Keywords

• nonn
• base
• more

Comments

11 | 5*10^(4*m) + 5*10^(2*m) + 1. So a(n) is odd for n > 1.

Examples

			           11 is prime ==> a(1) = 0.
          551 = 19 * 29.
        50501 = 11 * 4591.
      5005001 is prime ==> a(2) = 3.
    500050001 = 11 * 61 * 745231.
  50000500001 is prime ==> a(3) = 5.
5000005000001 = 11 * 31 * 1801 * 8141461.
		

Crossrefs

Cf. A309739.

Programs

PARI

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

Extensions

a(5)-a(6) from Michael S. Branicky, Sep 03 2024