OFFSET
1,2
COMMENTS
Numbers of this form divide 10^(4k)+4.
a(10) > 5000. - Jon E. Schoenfield, Dec 16 2017
EXAMPLE
61, 5101, 501001, 50010001, and 50000000010000000001 are prime, while 5000100001=13*1597*240841, 500001000001=2357*212134493, 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(8)-a(9) from Michel Marcus, Dec 13 2017
a(10) from Michael S. Branicky, Aug 16 2024
STATUS
approved