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

0,2

Links

Table of n, a(n) for n=0..14.

Example

a(1) = 5 because 2^(3*2^1)-5 = 2^6-5 = 59 is the largest safe prime less than 64.

Prog

(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

Crossrefs

Cf. A005385 (safe primes), A057821, A181356.

Sequence in context: A274002 A286678 A081479 * A318751 A096996 A256236

Adjacent sequences: A335310 A335311 A335312 * A335314 A335315 A335316

Keyword

nonn,more

Author

Artsiom Palkounikau, Jun 01 2020

Extensions

a(13) from Artsiom Palkounikau, Oct 13 2021

a(14) from Mark Andreas, Jun 06 2022

Status

approved