A281992 Numbers k such that (2*10^k - 143)/3 is prime.

2, 3, 4, 9, 12, 19, 48, 54, 90, 97, 104, 174, 349, 385, 1020, 1294, 1737, 2430, 9040, 14173, 15604, 17943, 37447, 149803, 164043

Offset

1,1

Comments

For k>1, numbers such that k-2 occurrences of the digit 6 followed by the digits 19 is prime (see Example section).

a(26) > 2*10^5.

Links

Table of n, a(n) for n=1..25.

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

Makoto Kamada, Search for 6w19.

Example

3 is in this sequence because (2*10^3 - 143)/3 = 619 is prime.
Initial terms and primes associated:
a(1) = 2, 19;
a(2) = 3, 619;
a(3) = 4, 6619;
a(4) = 9, 666666619;
a(5) = 12, 666666666619; etc.

Mathematica

Select[Range[2, 100000], PrimeQ[(2*10^# - 143)/3] &]

Crossrefs

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

Sequence in context: A145772 A027866 A283596 * A100902 A283447 A034793

Adjacent sequences: A281989 A281990 A281991 * A281993 A281994 A281995

Keyword

nonn,more,hard

Author

Robert Price, Feb 04 2017

Extensions

a(24)-a(25) from Robert Price, Mar 14 2018

Status

approved