A119624 - OEIS

%I #36 Apr 03 2018 06:26:51

%S 1,1,1,0,1,1,0,1,1,0,1,3,0,1,1,0,47,1,0,1,1,0,1,2,0,1,2,0,1,1,0,3,1,0,

%T 1,1,0,2729,1,0,1,2,0,1,2,0,175,1,0,1,1,0,1,1,0,1,3,0,3,3,0,43,1,0,1,

%U 2,0,1,1,0,3,2,0,1,1,0,3,1,0,11,1,0,1,4,0,1,2,0,1,1,0,3,2,0,1,1,0,1,1,0

%N Least k>0 such that, for n>1, 2*n^k + 1 is prime; or 0 if no such prime possible as 2*n^k + 1 is 0 mod(3).

%H Robert Israel, <a href="/A119624/b119624.txt">Table of n, a(n) for n = 1..217</a>

%p f:= proc(n) local k;

%p if n mod 3 = 1 then return 0 fi;

%p   if n mod 3 = 2 then r:= 2 else r:= 1 fi;

%p   for k from 1 by r do if isprime(2*n^k+1) then return k fi od

%p end proc:

%p f(1):= 1:

%p map(f, [$1..100]); # _Robert Israel_, Apr 02 2018

%t f[n_] := Block[{k = 0}, If[Mod[n, 3] != 1, k = 1; While[ ! PrimeQ[2*n^k + 1], k++ ]; ]; k ]; Table[f[n], {n, 2, 100}] (* _Ray Chandler_, Jun 08 2006 *)

%t Table[If[n>1 && Mod[n,3]==1, 0, k=1; While[ !PrimeQ[2n^k+1], k++ ]; k], {n,100}] (* _T. D. Noe_, Jun 08 2006 *)

%o (PARI) a(n) = if(n%3==1, 0, for(k=1, 2^24, if(ispseudoprime(2*n^k+1),return(k)))) \\ _Eric Chen_, Mar 20 2015

%Y Cf. A119591, A253178.

%K nonn

%O 1,12

%A _Pierre CAMI_, Jun 08 2006

%E Extended by _Ray Chandler_ and _T. D. Noe_, Jun 08 2006