OFFSET
1,3
EXAMPLE
13 is prime. ==> a(1) = 0.
661 is prime. ==> a(2) = 1.
60601 is prime. ==> a(3) = 2.
6006001 = 2027 * 2963.
600060001 is prime. ==> a(4) = 4.
PROG
(PARI) for(k=0, 1e3, if(ispseudoprime(6*100^k+6*10^k+1), print1(k", ")))
CROSSREFS
KEYWORD
nonn,more,base
AUTHOR
Seiichi Manyama, Aug 15 2019
EXTENSIONS
a(8)-a(9) from Michael S. Branicky, Sep 03 2024
STATUS
approved