OFFSET
1,3
COMMENTS
Numbers of this form divide 8*(10^3k)-1.
a(12) > 50000. - Robert Price, Apr 11 2018
EXAMPLE
7, 421, 4002001, and 40000200001 are prime, while 40201=7*5743, 400020001=31*12903871, etc.
MAPLE
select(k->isprime(4*10^(2*k)+2*(10^k)+1), [$0..1000]); # Muniru A Asiru, Apr 11 2018
MATHEMATICA
Select[Range[1500], PrimeQ[4 (10^(2 #)) + 2 (10^#) + 1] &] (* Michael De Vlieger, Dec 30 2017 *)
PROG
(PARI) isok(k) = isprime(4*10^(2*k)+2*(10^k)+1); \\ Michel Marcus, Dec 30 2017
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Patrick A. Thomas, Dec 30 2017
EXTENSIONS
a(6)-a(10) from Michael De Vlieger, Dec 30 2017
a(11) from Jon E. Schoenfield, Jan 01 2018
STATUS
approved