A293280 Numbers k such that (28*10^k - 31)/3 is prime.

1, 3, 4, 13, 15, 24, 28, 48, 233, 558, 621, 1498, 1810, 2788, 3532, 3987, 4791, 7972, 12221, 13714, 15940, 41759, 46956, 47827, 51651, 102142

Offset

1,2

Comments

For k > 1, numbers k such that the digit 9 followed by k-2 occurrences of the digit 3 followed by the digits 23 is prime (see Example section).

a(27) > 2*10^5.

Links

Table of n, a(n) for n=1..26.

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

Makoto Kamada, Search for 93w23.

Example

3 is in this sequence because (28*10^3 - 31)/3 = 9323 is prime.
Initial terms and associated primes:
a(1) = 1, 83;
a(2) = 3, 9323;
a(3) = 4, 93323;
a(4) = 13, 93333333333323;
a(5) = 15, 9333333333333323; etc.

Mathematica

Select[Range[1, 100000], PrimeQ[(28*10^# - 31)/3] &]

Crossrefs

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

Sequence in context: A105074 A208847 A278426 * A087884 A057570 A024853

Adjacent sequences: A293277 A293278 A293279 * A293281 A293282 A293283

Keyword

nonn,more,hard

Author

Robert Price, Oct 04 2017

Extensions

a(26) from Robert Price, Dec 08 2019

Status

approved