A123368 Numbers k such that 10*(10^(k+1) + 10^k - 1) + 7 is prime.

0, 1, 4, 5, 7, 22, 23, 40, 65, 73, 169, 271, 802, 1451, 2623, 4107, 7315, 7583, 7747, 8407, 9841, 19647, 23923, 24825, 30197, 35079

Offset

1,3

Links

Table of n, a(n) for n=1..26.

Example

107, 1097, 1099997, 10999997 are prime so terms are 0, 1, 4, 5.

Mathematica

Join[{0}, Select[Range[3000], PrimeQ[10*(10^(# + 1) + 10^# - 1) + 7] &]] (* G. C. Greubel, Oct 13 2017 *)

Prog

(PARI) g(n) = for(x=0, n, y=10*(10^(x+1)+10^x-1)+7; if(ispseudoprime(y), print1(x", ")))

Crossrefs

Sequence in context: A292321 A049896 A029520 * A292164 A178439 A214584

Adjacent sequences: A123365 A123366 A123367 * A123369 A123370 A123371

Keyword

more,nonn

Author

Cino Hilliard, Oct 12 2006

Extensions

More terms from Ryan Propper, Jan 11 2008

a(22)-a(24) from Michael S. Branicky, Jun 22 2023

a(25)-a(26) from Michael S. Branicky, Oct 30 2024

Status

approved