A247479 Smallest odd k > 1 such that k*2^n+1 is a prime number.

3, 3, 5, 7, 3, 3, 5, 3, 15, 13, 9, 3, 5, 7, 5, 21, 9, 3, 11, 7, 11, 25, 45, 45, 5, 7, 15, 13, 23, 3, 35, 43, 9, 75, 59, 3, 15, 15, 5, 27, 3, 9, 9, 15, 35, 19, 27, 15, 23, 7, 17, 7, 51, 49, 5, 27, 29, 99, 27, 31, 53, 105, 9, 25, 9, 3, 9, 31, 23

Offset: 1

Author

Pierre CAMI, Dec 01 2014

Keywords

• nonn

Comments

Differs from A057778 only where n is related to a Fermat prime (A019434). - R. J. Mathar, Dec 02 2014

Records: 3, 5, 7, 15, 21, 25, 45, 75, 99, 105, 127, 249, 321, 363, 411, 421, 535, 823, 1383, 1875, 2375, 2443, 2865, 4063, 4141, 4239, 4623, 5175, 5469, 14319, 15979, 17817, 25925, 30487, 39741, 48055, 49709, 50721, 55367, ... . - Robert G. Wilson v, Feb 02 2015

Links

• Robert G. Wilson v, Table of n, a(n) for n = 1..10111 (first 5150 terms from Pierre CAMI)

Crossrefs

Cf. A035050, A057778, A247202, A253398.

Programs

Maple

A247479:= proc(n) local k;
      for k from 3 by 2 do if isprime(k*2^n+1) then return k fi od
   end proc:
seq(A247479(n),n=1..100); # Robert Israel, Dec 01 2014

Mathematica

f[n_] := Block[{k = 3, p = 2^n}, While[ !PrimeQ[k*p + 1], k += 2]; k]; Array[f, 70] (* Robert G. Wilson v, Jan 29 2015 *)

PARI

a(n) = {k = 3; while (! isprime(k*2^n+1), k += 2); k;} \\ Michel Marcus, Dec 01 2014