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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 November 28 18:51 EST 2022. Contains 358421 sequences. (Running on oeis4.)