cp's OEIS Frontend

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

%I A247479 #58 Mar 02 2019 02:02:01
%S A247479 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,
%T A247479 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,
%U A247479 27,29,99,27,31,53,105,9,25,9,3,9,31,23
%N A247479 Smallest odd k > 1 such that k*2^n+1 is a prime number.
%C A247479 Differs from A057778 only where n is related to a Fermat prime (A019434). - _R. J. Mathar_, Dec 02 2014
%C A247479 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
%H A247479 Robert G. Wilson v, <a href="/A247479/b247479.txt">Table of n, a(n) for n = 1..10111</a> (first 5150 terms from Pierre CAMI)
%p A247479 A247479:= proc(n) local k;
%p A247479       for k from 3 by 2 do if isprime(k*2^n+1) then return k fi od
%p A247479    end proc:
%p A247479 seq(A247479(n),n=1..100); # _Robert Israel_, Dec 01 2014
%t A247479 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 *)
%o A247479 (PFGW & SCRIPT)
%o A247479 SCRIPT
%o A247479 DIM k
%o A247479 DIM n,0
%o A247479 DIMS t
%o A247479 OPENFILEOUT myf,a(n).txt
%o A247479 LABEL loop1
%o A247479 SET n,n+1
%o A247479 SET k,1
%o A247479 LABEL loop2
%o A247479 SET k,k+2
%o A247479 SETS t,%d,%d\,;n;k
%o A247479 PRP k*2^n+1,t
%o A247479 IF ISPRP THEN WRITE myf,t
%o A247479 IF ISPRP THEN GOTO loop1
%o A247479 GOTO loop2
%o A247479 (PARI) a(n) = {k = 3; while (! isprime(k*2^n+1), k += 2); k;} \\ _Michel Marcus_, Dec 01 2014
%Y A247479 Cf. A035050, A057778, A247202, A253398.
%K A247479 nonn
%O A247479 1,1
%A A247479 _Pierre CAMI_, Dec 01 2014