A293593 Numbers k such that (26*10^k - 413)/9 is prime.

3, 4, 6, 12, 15, 19, 36, 40, 484, 576, 2328, 2841, 4008, 4878, 5190, 10149, 10383, 11982, 21082, 46324, 61156, 83638, 125886

Offset

1,1

Comments

For k > 1, numbers k such that the digit 2 followed by k-2 occurrences of the digit 8 followed by the digits 43 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 28w43.

Example

3 is in this sequence because (26*10^3 - 413)/9 = 2843 is prime.
Initial terms and associated primes:
a(1) = 3, 2843;
a(2) = 4, 28843;
a(3) = 6, 2888843;
a(4) = 12, 2888888888843;
a(5) = 15, 2888888888888843; etc.

Mathematica

Select[Range[2, 100000], PrimeQ[(26*10^# - 413)/9] &]

Crossrefs

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

Sequence in context: A250985 A051592 A044843 * A024373 A111159 A210523

Adjacent sequences: A293590 A293591 A293592 * A293594 A293595 A293596

Keyword

nonn,more,hard

Author

Robert Price, Oct 12 2017

Extensions

a(23) from Robert Price, May 08 2018

Status

approved