login
A119624
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).
3
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, 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, 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
OFFSET
1,12
LINKS
MAPLE
f:= proc(n) local k;
if n mod 3 = 1 then return 0 fi;
if n mod 3 = 2 then r:= 2 else r:= 1 fi;
for k from 1 by r do if isprime(2*n^k+1) then return k fi od
end proc:
f(1):= 1:
map(f, [$1..100]); # Robert Israel, Apr 02 2018
MATHEMATICA
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 *)
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 *)
PROG
(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
CROSSREFS
Sequence in context: A147985 A147987 A036860 * A213727 A367452 A119612
KEYWORD
nonn
AUTHOR
Pierre CAMI, Jun 08 2006
EXTENSIONS
Extended by Ray Chandler and T. D. Noe, Jun 08 2006
STATUS
approved