A309744 Numbers k such that 81*10^(2*k) + 9*10^k + 1 is prime.

1, 3, 17, 31, 7101, 14183

Offset: 1

Author

Seiichi Manyama, Aug 15 2019

Keywords

• nonn
• more
• base

Examples

			                  91 = 7 * 13.
                8191 is prime ==> a(1) = 0.
              810901 = 7^2 * 13 * 19 * 67.
            81009001 is prime ==> a(2) = 3.
          8100090001 = 73 * 271 * 331 * 1237.
        810000900001 = 1471 * 550646431.
      81000009000001 = 7 * 13 * 613 * 757 * 1129 * 1699.
    8100000090000001 = 31 * 439 * 595194363289.
  810000000900000001 = 7 * 13 * 157 * 193 * 4243 * 4969 * 13933.
81000000009000000001 = 271 * 298892988963099631.
		

Crossrefs

Cf. A309738, A309743.

Programs

PARI

for(k=0, 1e3, if(ispseudoprime(81*100^k+9*10^k+1), print1(k", ")))

Extensions

a(6) from Michael S. Branicky, Sep 05 2024