A293189 Numbers k such that (76*10^k - 211)/9 is prime.

1, 2, 4, 5, 8, 13, 41, 55, 82, 188, 296, 320, 464, 509, 1036, 1169, 1808, 2141, 3842, 12274, 23141, 76547, 83816, 85847, 154013

Offset

1,2

Comments

For k > 1, numbers k such that the digit 8 followed by k-2 occurrences of the digit 4 followed by the digits 21 is prime (see Example section).

a(26) > 2*10^5.

Links

Table of n, a(n) for n=1..25.

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

Makoto Kamada, Search for 84w21.

Example

4 is in this sequence because (76*10^4 - 211)/9 = 84421 is prime.
Initial terms and associated primes:
a(1) = 1, 61;
a(2) = 2, 821;
a(3) = 4, 84421;
a(4) = 5, 844421;
a(5) = 8, 844444421; etc.

Mathematica

Select[Range[1, 100000], PrimeQ[(76*10^# - 211)/9] &]

Crossrefs

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

Sequence in context: A327045 A288668 A264800 * A278695 A105134 A226791

Adjacent sequences: A293186 A293187 A293188 * A293190 A293191 A293192

Keyword

nonn,more,hard

Author

Robert Price, Oct 01 2017

Extensions

a(25) from Robert Price, Aug 31 2019

Status

approved