A296445 Numbers k such that 10^(2k)/2 - 10^k + 1 is prime.

1, 5, 10, 12, 24, 48, 960, 2217, 13045, 13140

Offset

1,2

Comments

Numbers of this form divide 10^(4k)+4.

Links

Table of n, a(n) for n=1..10.

Example

41, 4999900001, 49999999990000000001, and 499999999999000000000001 are prime, while 4901=13^2*29, 499001=17*149*197, etc.

Mathematica

ParallelMap[If[PrimeQ[10^(2 #)/2 - 10^# + 1], #, Nothing] &, Range@ 4000] (* Robert G. Wilson v, Dec 13 2017 *)

Prog

(PARI) isok(k) = isprime(10^(2*k)/2-10^k+1); \\ Michel Marcus, Dec 13 2017

Crossrefs

See A296446 for 10^(2k)/2 + 10^k + 1.

Sequence in context: A103676 A313375 A276606 * A313376 A129846 A194464

Adjacent sequences: A296442 A296443 A296444 * A296446 A296447 A296448

Keyword

nonn,more

Author

Patrick A. Thomas, Dec 13 2017

Extensions

a(7) from Michel Marcus, Dec 13 2017

a(8) from Robert G. Wilson v, Dec 13 2017

a(9)-a(10) from Michael S. Branicky, Apr 09 2024

Status

approved