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A181356 Smallest k such that 2^(2^n) - k is a safe prime. 1
5, 29, 269, 209, 1469, 15449, 36113, 38117, 1093337, 1942289, 10895177, 43644929, 364486013, 718982153 (list; graph; refs; listen; history; text; internal format)
OFFSET

2,1

COMMENTS

These numbers are useful as Diffie-Hellman moduli.

LINKS

Table of n, a(n) for n=2..15.

Mersenne Forum, CRUS-like sieving challenge

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

Cf. A058220, A005385.

Sequence in context: A144015 A326986 A228387 * A177440 A292567 A332517

Adjacent sequences:  A181353 A181354 A181355 * A181357 A181358 A181359

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

STATUS

approved

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Last modified July 26 09:58 EDT 2021. Contains 346294 sequences. (Running on oeis4.)