A284595 Numbers k such that (349*10^k - 7)/9 is prime.

2, 12, 21, 30, 63, 86, 140, 173, 308, 318, 402, 410, 441, 492, 2186, 4580, 5100, 5208, 5810, 7476, 21677, 31766, 45357, 48036, 66720

Offset

1,1

Comments

For k > 1, numbers k such that the digits 38 followed by k occurrences of the digit 7 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 387w.

Example

2 is in this sequence because (349*10^2 - 7)/9 = 3877 is prime.
Initial terms and associated primes:
a(1) = 2, 3877;
a(2) = 12, 38777777777777;
a(3) = 21, 38777777777777777777777;
a(4) = 30, 38777777777777777777777777777777;
a(5) = 63, 38777777777777777777777777777777777777777777777777777777777777777; etc.

Mathematica

Select[Range[0, 100000], PrimeQ[(349*10^# - 7)/9] &]

Crossrefs

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

Sequence in context: A304813 A279424 A073041 * A053898 A058413 A053890

Adjacent sequences: A284592 A284593 A284594 * A284596 A284597 A284598

Keyword

nonn,more,hard

Author

Robert Price, Mar 29 2017

Status

approved