A297421 Numbers k such that (10^(2k) + 2*(10^k) + 4)/4 is prime.

1, 2, 3, 15, 175, 449, 16563

Offset

1,2

Comments

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

a(7) > 3000. - Jon E. Schoenfield, Jan 02 2018

a(8) > 50000. - Robert Price, Apr 11 2018

Links

Table of n, a(n) for n=1..7.

Example

31, 2551, 250501, and 250000000000000500000000000001 are prime, while 25005001=7*79*103*439, 2500050001=19*2269*57991, etc.

Maple

select(k->isprime((10^(2*k)+2*(10^k)+4)/4), [$1..1000]); # Muniru A Asiru, Apr 11 2018

Mathematica

Select[Range[10^3], PrimeQ[(10^(2 #) + 2 (10^#) + 4)/4] &] (* Michael De Vlieger, Dec 30 2017 *)

Prog

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

Crossrefs

Sequence in context: A165657 A091835 A131612 * A180723 A162102 A087975

Adjacent sequences: A297418 A297419 A297420 * A297422 A297423 A297424

Keyword

nonn,more

Author

Patrick A. Thomas, Dec 30 2017

Extensions

a(5)-a(6) from Michel Marcus, Dec 30 2017

a(7) from Robert Price, Apr 11 2018

Status

approved