A335313 Smallest k such that 2^(3*2^n) - k is a safe prime.
1, 5, 17, 317, 5297, 3449, 41213, 59057, 468857, 1503317, 1103717, 40207829, 154474973, 1162354373, 18153497
Offset: 0
Examples
a(1) = 5 because 2^(3*2^1)-5 = 2^6-5 = 59 is the largest safe prime less than 64.
Programs
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PARI
a(n) = {my(k=0); while (!(isprime(p=2^(3*2^n) - k) && isprime((p-1)/2)), k++); k;} \\ Michel Marcus, Jun 01 2020 -
Python
from sympy import isprime, prevprime def A335313(n): m = 2**(3*2**n) p = prevprime(m) while not isprime((p-1)//2): p = prevprime(p) return m-p # Chai Wah Wu, Jul 09 2020
Extensions
a(13) from Artsiom Palkounikau, Oct 13 2021
a(14) from Mark Andreas, Jun 06 2022