A297419 Numbers k such that (10^(2k) - 2*(10^k) + 4)/12 is prime.

1, 4, 5, 8, 12, 30, 34, 35, 77, 144, 204, 368, 1086, 6755, 7056, 17548

Offset

1,2

Comments

Numbers of this form divide 10^(3k)+8.

a(16) > 10360. - Robert G. Wilson v, Jan 12 2018

a(17) > 50000. - Robert Price, Apr 15 2018

Links

Table of n, a(n) for n=1..16.

Example

7, 8331667, 833316667, and 833333316666667 are prime, while 817=19*43, 83167=7*109^2, and 83333166667=97*26209*32779.

Mathematica

Select[Range[1500], PrimeQ[(10^(2 #) - 2 (10^#) + 4)/12] &] (* Michael De Vlieger, Dec 30 2017 *)

Prog

(PARI) isok(k) = isprime((10^(2*k) - 2*(10^k) + 4)/12); \\ Michel Marcus, Dec 30 2017

Crossrefs

Sequence in context: A362560 A061765 A242274 * A230549 A133940 A174398

Adjacent sequences: A297416 A297417 A297418 * A297420 A297421 A297422

Keyword

nonn,more

Author

Patrick A. Thomas, Dec 30 2017

Extensions

a(10) from Michel Marcus, Dec 30 2017

a(11)-a(13) from Michael De Vlieger, Dec 30 2017

a(14)-a(15) from Robert G. Wilson v, Jan 08 2018

a(16) from Robert Price, Apr 15 2018

Status

approved