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

1,1

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)

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 *)

Prog

(PFGW & SCRIPT)
SCRIPT
DIM k
DIM n, 0
DIMS t
OPENFILEOUT myf, a(n).txt
LABEL loop1
SET n, n+1
SET k, 1
LABEL loop2
SET k, k+2
SETS t, %d, %d\,; n; k
PRP k*2^n+1, t
IF ISPRP THEN WRITE myf, t
IF ISPRP THEN GOTO loop1
GOTO loop2
(PARI) a(n) = {k = 3; while (! isprime(k*2^n+1), k += 2); k; } \\ Michel Marcus, Dec 01 2014

Crossrefs

Cf. A035050, A057778, A247202, A253398.

Sequence in context: A089874 A092035 A164914 * A134855 A335045 A110246

Adjacent sequences: A247476 A247477 A247478 * A247480 A247481 A247482

Keyword

nonn

Author

Pierre CAMI, Dec 01 2014

Status

approved