OFFSET
1,3
EXAMPLE
17 is prime. ==> a(1) = 0.
881 is prime. ==> a(2) = 1.
80801 = 7^2 * 17 * 97.
8008001 = 47 * 170383.
800080001 = 7 * 23 * 103 * 48247.
80000800001 = 71 * 1126771831.
8000008000001 is prime. ==> a(3) = 6.
PROG
(PARI) for(k=0, 1e3, if(ispseudoprime(8*100^k+8*10^k+1), print1(k", ")))
(Python)
from sympy import isprime
def afind(limit, startk=0):
for k in range(startk, limit+1):
if isprime(8*100**k + 8*10**k + 1): print(k, end=", ")
afind(500) # Michael S. Branicky, Dec 12 2021
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Seiichi Manyama, Aug 15 2019
EXTENSIONS
a(11) from Michael S. Branicky, Dec 12 2021
a(12) from Michael S. Branicky, Apr 16 2023
STATUS
approved