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A126715 a(n) is the smallest odd prime p such that p*2^n - 1 is prime. 6
3, 3, 3, 3, 3, 7, 3, 3, 5, 7, 5, 3, 5, 31, 5, 79, 17, 7, 3, 61, 17, 7, 83, 13, 83, 61, 11, 193, 83, 7, 521, 43, 5, 31, 3, 31, 17, 31, 3, 61, 107, 19, 53, 3, 557, 7, 23, 31, 5, 19, 11, 1033, 89, 307, 5, 3, 563, 79, 83, 733, 17, 79, 53, 61, 3, 67, 257, 43, 179, 139, 47, 73, 5, 421, 113 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,1

COMMENTS

By Xylouris' version of Linnik's theorem, a(n) << 2^(5.2n). - Charles R Greathouse IV, Dec 28 2011

a(n) = prime(k) for some k < 5*n, for the even prime 2*2^n-1 is prime for n = prime(k)-1. - Pierre CAMI, Jul 20 2014

LINKS

T. D. Noe and Pierre CAMI, Table of n, a(n) for n = 0..10000 (first 2501 terms from T. D. Noe)

MATHEMATICA

f[n_] := Block[{k = 2}, While[ !PrimeQ[ Prime[k]*2^n - 1], k++ ]; Prime@k]; Table[f@n, {n, 0, 74}] (* Robert G. Wilson v *)

PROG

(PARI) a(n) = p=3; t=2^n; while(!isprime(p*t-1), p=nextprime(p+1)); p \\ Colin Barker, Jul 22 2014

CROSSREFS

Sequence in context: A178154 A270774 A263144 * A158805 A163469 A105121

Adjacent sequences:  A126712 A126713 A126714 * A126716 A126717 A126718

KEYWORD

nonn

AUTHOR

Pierre CAMI, Feb 13 2007

EXTENSIONS

More terms from Robert G. Wilson v, Feb 16 2007

Entries checked by N. J. A. Sloane, Mar 02 2007 and some errors corrected.

STATUS

approved

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Last modified April 18 12:45 EDT 2021. Contains 343088 sequences. (Running on oeis4.)