A126715 a(n) is the smallest odd prime p such that p*2^n - 1 is prime.

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

Offset

0,1

Comments

By Xylouris's 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. - Pierre CAMI, Jul 20 2014

Links

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, Feb 16 2007 *)

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

Status

approved