A272830 Numbers k such that (8*10^k - 29)/3 is prime.

1, 2, 3, 8, 9, 10, 16, 31, 35, 79, 179, 196, 239, 376, 515, 728, 812, 1154, 2000, 2379, 2485, 3523, 3987, 5221, 5257, 5739, 17863, 59127, 106454, 125894

Offset: 1

Author

Robert Price, May 07 2016

Keywords

• nonn
• more

Comments

For k > 1, numbers k such that the digit 2 followed by k-2 occurrences of the digit 6 followed by the digits 57 is prime (see Example section).

a(31) > 2*10^5.

Examples

			3 is in this sequence because (8*10^3 - 29)/3 = 2657 is prime.
Initial terms and associated primes:
a(1) = 1, 17;
a(2) = 2, 257;
a(3) = 3, 2657;
a(4) = 8, 266666657;
a(5) = 9, 2666666657, etc.
		

Links

• Makoto Kamada, Factorization of near-repdigit-related numbers.
• Makoto Kamada, Search for 26w57.

Crossrefs

Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.

Programs

Mathematica

Select[Range[0, 100000], PrimeQ[(8*10^# - 29)/3] &]

PARI

is(n)=ispseudoprime((8*10^n-29)/3) \\ Charles R Greathouse IV, Jun 13 2017

Extensions

a(29)-a(30) from Robert Price, Jul 03 2018