A287684 Numbers k such that 9*10^k - 23 is prime.

1, 2, 4, 13, 16, 19, 25, 44, 55, 64, 69, 183, 241, 290, 345, 398, 410, 423, 860, 1287, 2398, 2553, 3109

Offset

1,2

Comments

For k > 1, numbers k such that the digit 8 followed by k-2 occurrences of the digit 9 followed by the digits 77 is prime (see Example section).

a(24) > 2*10^5.

Links

Table of n, a(n) for n=1..23.

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

Makoto Kamada, Search for 89w77.

Example

4 is in this sequence because 9*10^4 - 23 = 89977 is prime.
Initial terms and associated primes:
a(1) = 1, 67;
a(2) = 2, 877;
a(3) = 4, 89977;
a(4) = 13, 89999999999977;
a(5) = 16, 89999999999999977; etc.

Mathematica

Select[Range[1, 100000], PrimeQ[9*10^# - 23] &]

Crossrefs

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

Sequence in context: A024580 A074097 A276481 * A033628 A077319 A291901

Adjacent sequences: A287681 A287682 A287683 * A287685 A287686 A287687

Keyword

nonn,more,hard

Author

Robert Price, May 29 2017

Status

approved