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URL: https://oeis.org/A181356

⇱ A181356 - OEIS

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A181356
Smallest k such that 2^(2^n) - k is a safe prime.
6
5, 29, 269, 209, 1469, 15449, 36113, 38117, 1093337, 1942289, 10895177, 43644929, 364486013, 718982153, 2356107437
OFFSET
2,1
COMMENTS
These numbers are useful as Diffie-Hellman moduli.
EXAMPLE
a(2)=5 because 2^(2^2) - 5 = 11 is the largest safe prime less than 16.
PROG
(PARI) a(n) = {k = 1; pow2 = 2^(2^n); while (! (isprime(pow2 - k) && isprime((pow2 - k - 1)/2)), k +=2; ); k; } \\ Michel Marcus, Aug 16 2013
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Ken Takusagawa, Jan 27 2011
EXTENSIONS
a(14) from Ken Takusagawa, May 31 2011
a(15) from Gary Barnes, Oct 26 2011
a(16) from Mark Andreas, Jan 23 2023
STATUS
approved