A272271 Numbers k such that 7*10^k - 23 is prime.

1, 2, 3, 23, 29, 34, 35, 38, 52, 57, 61, 82, 186, 209, 251, 366, 394, 426, 786, 979, 1382, 2037, 4557, 8995, 12774, 19170, 21828, 23259, 32003, 41831, 44999, 56785, 76483, 97987, 110468

Offset: 1

Author

Robert Price, Apr 24 2016

Keywords

• nonn
• more

Comments

For k > 1, numbers k such that the digit 6 followed by k-2 occurrences of the digit 9 followed by the digits 77 is prime (see Example section).

a(36) > 3*10^5.

Examples

			3 is in this sequence because 7*10^3 - 23 = 6977 is prime.
Initial terms and associated primes:
a(1) = 1, 47;
a(2) = 2, 677;
a(3) = 3, 6977;
a(4) = 23, 699999999999999999999977;
a(5) = 29, 699999999999999999999999999977, etc.
		

Links

• Makoto Kamada, Factorization of near-repdigit-related numbers.
• Makoto Kamada, Search for 69w77.

Crossrefs

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

Programs

Mathematica

Select[Range[0, 100000], PrimeQ[7*10^# - 23] &]

PARI

is(n)=ispseudoprime(7*10^n - 23) \\ Charles R Greathouse IV, Jun 13 2017

Extensions

a(35) from Robert Price, Jul 27 2019