A281734 Numbers k such that (2*10^k + 529)/9 is prime.

0, 1, 3, 4, 7, 9, 16, 18, 21, 33, 34, 45, 49, 57, 567, 595, 685, 867, 1867, 4204, 5311, 11493, 13923, 19116, 30471, 32038, 34551, 99408, 113631, 134364, 195399

Offset

1,3

Comments

For k > 1, numbers k such that k-2 occurrences of the digit 2 followed by the digits 81 is prime (see Example section).

a(32) > 2*10^5.

Links

Table of n, a(n) for n=1..31.

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

Makoto Kamada, Search for 2w81.

Example

3 is in this sequence because (2*10^3 + 529)/9 = 281 is prime.
Initial terms and associated primes:
a(1) = 0, 59;
a(2) = 1, 61;
a(3) = 3, 281;
a(4) = 4, 2281;
a(5) = 7, 2222281; etc.

Mathematica

Select[Range[0, 100000], PrimeQ[(2*10^# + 529)/9] &]

Crossrefs

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

Sequence in context: A086772 A301767 A169898 * A086336 A237258 A108796

Adjacent sequences: A281731 A281732 A281733 * A281735 A281736 A281737

Keyword

nonn,more,hard

Author

Robert Price, Jan 28 2017

Extensions

a(29)-a(31) from Robert Price, Jan 02 2018

Status

approved