A271109 Numbers k such that (5 * 10^k - 119)/3 is prime.

2, 3, 5, 6, 8, 11, 26, 33, 35, 41, 69, 73, 204, 230, 295, 381, 392, 537, 776, 1187, 2187, 2426, 4182, 4589, 5841, 6107, 11513, 13431, 28901, 56256, 65203, 66613, 82085, 91707, 126871, 140281

Offset: 1

Author

Robert Price, Apr 05 2016

Keywords

• nonn
• more

Comments

For k > 1, numbers k such that the digit 1 followed by k - 2 occurrences of the digit 6 followed by the digits 27 is prime (see Example section).

a(37) > 2*10^5.

Examples

			3 is in this sequence because (5*10^3 - 119)/3 = 1627 is prime.
Initial terms and associated primes:
a(1) = 2, 127;
a(2) = 3, 1627;
a(3) = 5, 166627;
a(4) = 6, 1666627;
a(5) = 8, 166666627, etc.
		

Links

• Makoto Kamada, Search for 16w27.

Crossrefs

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

Programs

Mathematica

Select[Range[10^5], PrimeQ[(5 * 10^# - 119)/3] &]

PARI

is(n)=ispseudoprime((5*10^n-119)/3) \\ Charles R Greathouse IV, Jun 13 2017

Extensions

a(35)-a(36) from Robert Price, Mar 29 2018