OFFSET
1,2
COMMENTS
Numbers of this form divide 8*(10^3k)+1.
a(11) > 3000. - Jon E. Schoenfield, Jan 01 2018
a(12) > 50000. - Robert Price, Apr 15 2018
EXAMPLE
127 and 13267 are prime, while 1332667=7*61*3121, 133326667=19*937*7489, and 13333333266666667=6247609*2134149763.
MATHEMATICA
Select[Range[1500], PrimeQ[(4 (10^(2 #)) - 2 (10^#) + 1)/3] &] (* Michael De Vlieger, Dec 30 2017 *)
PROG
(PARI) isok(k) = isprime((4*10^(2*k) - 2*(10^k) + 1)/3); \\ Michel Marcus, Dec 30 2017
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Patrick A. Thomas, Dec 30 2017
EXTENSIONS
a(3)-a(5) from Michel Marcus, Dec 30 2017
a(6)-a(9) from Michael De Vlieger, Dec 30 2017
a(10) from Jon E. Schoenfield, Jan 01 2018
a(11) from Robert Price, Apr 15 2018
STATUS
approved