A281167 Numbers k such that (23*10^k - 143)/3 is prime.

1, 2, 6, 7, 14, 22, 31, 33, 42, 51, 799, 934, 1996, 3372, 3570, 3883, 4417, 4747, 4965, 5055, 5647, 7183, 19702, 42124, 180862, 187164

Offset

1,2

Comments

For k > 1, numbers k such that the digit 7 followed by k-2 occurrences of the digit 6 followed by the digits 19 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 76w19.

Example

2 is in this sequence because (23*10^2 - 143) / 3 = 719 is prime.
Initial terms and associated primes:
a(1) = 1, 29;
a(2) = 2, 719;
a(3) = 6, 7666619;
a(4) = 7, 76666619;
a(5) = 14, 766666666666619; etc.

Mathematica

Select[Range[0, 100000], PrimeQ[(23*10^# - 143) / 3] &]

Crossrefs

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

Sequence in context: A176279 A376296 A265739 * A210660 A344343 A226965

Adjacent sequences: A281164 A281165 A281166 * A281168 A281169 A281170

Keyword

nonn,more,hard

Author

Robert Price, Jan 16 2017

Extensions

a(25)-a(26) from Robert Price, Dec 04 2019

Status

approved