A295129 Numbers k such that (89*10^k - 179)/9 is prime.

1, 4, 10, 12, 15, 52, 72, 150, 160, 271, 732, 1566, 2673, 4840, 7818, 9016, 9441, 9448, 11190, 15792, 64504, 80650, 115030, 140008

Offset

1,2

Comments

For k > 1, numbers k such that the digit 9 followed by k-2 occurrences of the digit 8 followed by the digits 69 is prime (see Example section).

a(25) > 2*10^5.

Links

Table of n, a(n) for n=1..24.

Makoto Kamada, Factorization of near-repdigit-related numbers.

Makoto Kamada, Search for 98w69.

Example

2 is in this sequence because (89*10^4 - 179)/9 = 98869 is prime.
Initial terms and associated primes:
a(1) = 1, 79;
a(2) = 4, 98869;
a(3) = 10, 98888888869;
a(4) = 12, 9888888888869;
a(5) = 15, 9888888888888869; etc.

Mathematica

Select[Range[1, 100000], PrimeQ[(89*10^# - 179)/9] &]

Prog

(PARI) isok(k) = isprime((89*10^k - 179)/9); \\ Michel Marcus, Nov 15 2017

Crossrefs

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

Sequence in context: A299634 A181053 A239055 * A287338 A077654 A095275

Adjacent sequences: A295126 A295127 A295128 * A295130 A295131 A295132

Keyword

nonn,more,hard,changed

Author

Robert Price, Nov 15 2017

Extensions

a(23)-a(24) from Robert Price, Dec 16 2019

Status

approved