A309742 - OEIS

A309742

Numbers k such that 8*10^(2*k) + 8*10^k + 1 are prime.

0, 1, 6, 11, 23, 297, 474, 1121, 2531, 3573, 5437, 5919

(list; graph; refs; listen; history; text; internal format)

OFFSET

1,3

LINKS

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

EXTENSIONS

a(11) from Michael S. Branicky, Dec 12 2021

a(12) from Michael S. Branicky, Apr 16 2023

STATUS

approved