OFFSET
2,1
COMMENTS
The values of k are in A189240.
LINKS
Amiram Eldar, Table of n, a(n) for n = 2..658
EXAMPLE
a(4) = 41 because 3^4 + 1 = 2 * 41 ; the smallest prime divisor of the form 2*k*n+1 is 41 = 2*5*4+1.
MATHEMATICA
Table[p=First/@FactorInteger[3^n+1]; Select[p, Mod[#1, n] == 1 &, 1][[1]],
{n, 2, 40}]
CROSSREFS
KEYWORD
nonn
AUTHOR
Michel Lagneau, Apr 19 2011
STATUS
approved