OFFSET
1,3
EXAMPLE
73 is prime ==> a(1) = 0.
6481 is prime ==> a(2) = 1.
640801 = 7 * 31 * 2953.
64008001 is prime ==> a(3) = 3.
6400080001 = 7 * 13441 * 68023.
640000800001 = 619 * 1033926979.
64000008000001 is prime ==> a(4) = 6.
PROG
(PARI) for(k=0, 1e3, if(ispseudoprime(64*100^k+8*10^k+1), print1(k", ")))
CROSSREFS
KEYWORD
nonn,more,base
AUTHOR
Seiichi Manyama, Aug 15 2019
EXTENSIONS
a(9) from Michael S. Branicky, Sep 04 2024
STATUS
approved