1,2

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

Table of n, a(n) for n=1..4.

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.

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

Cf. A309739.

Sequence in context: A101331 A232239 A087368 * A280035 A087670 A271390

Adjacent sequences: A309737 A309738 A309739 * A309741 A309742 A309743

nonn,base,more

Seiichi Manyama, Aug 15 2019

approved