OFFSET
1,2
COMMENTS
Numbers of this form divide 4*10^(4k)+1.
EXAMPLE
5, 20201, 2002001, 2000002000001, and 200000000020000000001 are prime, while 221=13*17, 200020001=569*351529, and 20000200001=17*29*1129*35933.
MATHEMATICA
ParallelMap[ If[ PrimeQ[2*10^(2 #) + 2*10^# + 1], #, Nothing] &, Range@ 6500] (* Robert G. Wilson v, Dec 13 2017 *)
PROG
(PARI) isok(k) = isprime(2*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(6)-a(7) from Michel Marcus, Dec 13 2017
a(8)-a(10) from Robert G. Wilson v, Dec 13 2017
a(1) = 0 inserted by Seiichi Manyama, Aug 15 2019
a(11) from Michael S. Branicky, Apr 16 2023
STATUS
approved