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A057821 a(n) = least nonnegative integer k such that 2^n - k is a safe prime. 0
1, 5, 9, 5, 21, 29, 9, 5, 9, 17, 45, 161, 165, 269, 285, 17, 45, 233, 9, 17, 321, 317, 633, 677, 405, 437, 189, 1385, 69, 209, 9, 641, 849, 137, 45, 401, 381, 437, 1965, 2201, 741, 1493, 573, 857, 1485, 5297, 2709, 161, 465, 473, 1269, 4805, 789 (list; graph; refs; listen; history; text; internal format)
OFFSET

3,2

COMMENTS

Previous name was: "Useful safe primes: a(n) = least nonnegative integer k such that 2^n - k is prime and (2^n-k-1)/2 is also prime". The resulting sequence of 2^n-k terms: 7, 11, 23, 59, 107, ..., are thus the largest safe primes smaller than 2^n (A243916), a subsequence of A005385. - Michel Marcus, Jan 08 2014

LINKS

Table of n, a(n) for n=3..55.

PROG

(PARI) a(n) = {k = 0; until (isprime(2^n-k) && isprime((2^n-k-1)/2), k++); return (k); } \\ Michel Marcus, Jun 29 2013

(Python)

from sympy import isprime

def a(n):

    k=0

    while True:

        k+=1

        if isprime(2**n - k) and isprime((2**n - k - 1)/2): return k

print [a(i) for i in xrange(3, 21)] # Indranil Ghosh, Jun 12 2017, after PARI code by Michel Marcus

CROSSREFS

Cf. A013603.

Sequence in context: A200283 A010720 A094389 * A133742 A134879 A051158

Adjacent sequences:  A057818 A057819 A057820 * A057822 A057823 A057824

KEYWORD

nonn

AUTHOR

Warren D. Smith, Nov 23 2000

STATUS

approved

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Last modified July 18 07:05 EDT 2019. Contains 325134 sequences. (Running on oeis4.)