OFFSET
1,2
COMMENTS
The numbers that follow the expression in the definition have this form: (3) concatenated k times and prepended to 17.
All terms are odd numbers (if k is even then one can use a^2 - b^2 = (a+b)*(a-b) to get a factorization).
EXAMPLE
3 appears because 33317 ('3' concatenated 3 times and prepended to '17') is prime.
MATHEMATICA
Select[Select[Range[2000], OddQ], PrimeQ[(10^(# + 2) - 49)/3] &] (* or *)
ParallelMap[If[OddQ[#] && PrimeQ[(10^(# + 2) - 49)/3], #, Nothing] &, Range[2000]]
PROG
(Magma) [n: n in [1..400]| IsPrime((10^(n+2) - 49) div 3)]; // Vincenzo Librandi, Nov 29 2015
(PARI) is(n)=ispseudoprime((10^(n+2)-49)/3) \\ Charles R Greathouse IV, Jun 13 2017
CROSSREFS
KEYWORD
nonn,base,hard,more
AUTHOR
Mikk Heidemaa, Nov 28 2015
EXTENSIONS
a(13)-a(14) by Mikk Heidemaa, Apr 28 2017
STATUS
approved