A293911 Numbers k such that (86*10^k - 311)/9 is prime.

1, 3, 6, 13, 31, 58, 76, 96, 147, 174, 306, 532, 1177, 3019, 5790, 5952, 7489, 10269, 10936, 20209, 28188, 55645, 71068, 89640

Offset

1,2

Comments

For k>1, numbers such that the digit 9 followed by k-2 occurrences of the digit 5 followed by the digits 21 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 95w21.

Example

3 is in this sequence because (86*10^3 - 311)/9 = 9521 is prime.
Initial terms and primes associated:
a(1) = 1, 61;
a(2) = 3, 9521;
a(3) = 6, 9555521;
a(4) = 13, 95555555555521;
a(5) = 31, 95555555555555555555555555555521; etc.

Mathematica

Select[Range[1, 100000], PrimeQ[(86*10^# - 311)/9] &]

Crossrefs

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

Sequence in context: A141353 A130582 A126296 * A018014 A358937 A162483

Adjacent sequences: A293908 A293909 A293910 * A293912 A293913 A293914

Keyword

nonn,more,hard

Author

Robert Price, Oct 19 2017

Status

approved