OFFSET
1,2
COMMENTS
Numbers of this form divide 10^(4k)+4.
EXAMPLE
41, 4999900001, 49999999990000000001, and 499999999999000000000001 are prime, while 4901=13^2*29, 499001=17*149*197, etc.
MATHEMATICA
ParallelMap[If[PrimeQ[10^(2 #)/2 - 10^# + 1], #, Nothing] &, Range@ 4000] (* Robert G. Wilson v, Dec 13 2017 *)
PROG
(PARI) isok(k) = isprime(10^(2*k)/2-10^k+1); \\ Michel Marcus, Dec 13 2017
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Patrick A. Thomas, Dec 13 2017
EXTENSIONS
a(7) from Michel Marcus, Dec 13 2017
a(8) from Robert G. Wilson v, Dec 13 2017
a(9)-a(10) from Michael S. Branicky, Apr 09 2024
STATUS
approved