A273783 Numbers k such that (86*10^k - 77)/9 is prime.

2, 3, 8, 9, 12, 14, 27, 32, 50, 80, 98, 99, 194, 237, 338, 828, 830, 1265, 2583, 3639, 5232, 5940, 9371, 10268, 13424, 26975, 36147, 60165, 69260, 93263

Offset: 1

Author

Robert Price, May 30 2016

Keywords

• nonn
• more

Comments

For k > 1, numbers k such that the digit 9 followed by k-2 occurrences of the digit 5 followed by the digits 47 is prime (see Example section).

a(31) > 2*10^5.

Examples

			3 is in this sequence because (86*10^3 - 77)/9 = 9547 is prime.
Initial terms and associated primes:
a(1) = 2, 947;
a(2) = 3, 9547;
a(3) = 8, 955555547;
a(4) = 9, 9555555547;
a(5) = 12, 9555555555547, etc.
		

Links

• Makoto Kamada, Factorization of near-repdigit-related numbers.
• Makoto Kamada, Search for 95w47.

Crossrefs

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

Programs

Mathematica

Select[Range[0, 100000], PrimeQ[(86*10^# - 77)/9] &]

PARI

is(n)=ispseudoprime((86*10^n - 77)/9) \\ Charles R Greathouse IV, Jun 08 2016