OFFSET
1,2
COMMENTS
Numbers of this form divide 4*10^(4k)+1.
a(8) > 5000. - Jon E. Schoenfield, Dec 16 2017
a(12) > 25000. - Michael S. Branicky, Aug 08 2024
EXAMPLE
181, 19801, 1999998000001, 199999980000001, and 19999999800000001 are prime, while 1998001=277*7213, 199980001=13*41*457*821, and 19999800001=53*5953*63389.
MATHEMATICA
ParallelMap[If[PrimeQ[2*10^(2 #) - 2*10^# + 1], #, Nothing] &, Range@ 4000] (* 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(11) from Michael S. Branicky, Mar 31 2023
STATUS
approved