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A309742
Numbers k such that 8*10^(2*k) + 8*10^k + 1 are prime.
2
0, 1, 6, 11, 23, 297, 474, 1121, 2531, 3573, 5437, 5919
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
Cf. A309739.
Sequence in context: A155449 A220154 A362441 * A362442 A372999 A063629
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