A293842 Numbers k such that 3*10^k - 31 is prime.

2, 3, 5, 10, 21, 101, 133, 187, 209, 363, 747, 1550, 3715, 3875, 6022, 9729, 10460, 12316, 14421, 19023, 50727

Offset

1,1

Comments

For k > 1, numbers k such that the digit 2 followed by k-2 occurrences of the digit 9 followed by the digits 69 is prime (see Example section).

a(22) > 2*10^5.

Links

Table of n, a(n) for n=1..21.

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

Makoto Kamada, Search for 29w69.

Example

3 is in this sequence because 3*10^3 - 31 = 2969 is prime.
Initial terms and associated primes:
a(1) = 2, 269;
a(2) = 3, 2969;
a(3) = 5, 299969;
a(4) = 10, 29999999969;
a(5) = 21, 2999999999999999999969; etc.

Mathematica

Select[Range[2, 100000], PrimeQ[3*10^# - 31] &]

Crossrefs

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

Sequence in context: A346970 A001646 A103595 * A014844 A307264 A329244

Adjacent sequences: A293839 A293840 A293841 * A293843 A293844 A293845

Keyword

nonn,more,hard

Author

Robert Price, Oct 18 2017

Status

approved