login
The OEIS is supported by the many generous donors to the OEIS Foundation.

 

Logo

Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 59th year, we have over 358,000 sequences, and we’ve crossed 10,300 citations (which often say “discovered thanks to the OEIS”).

Other ways to Give
Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
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

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recents
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

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