A290955 Numbers k such that (178*10^k - 7)/9 is prime.

0, 1, 3, 7, 12, 13, 21, 40, 67, 69, 132, 213, 259, 1056, 1639, 1653, 2913, 6183, 7086, 8466, 27475, 55390

Offset

1,3

Comments

Numbers k such that the digits 19 followed by k occurrences of the digit 7 is prime (see Example section).

a(23) > 2*10^5.

Links

Table of n, a(n) for n=1..22.

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

Makoto Kamada, Search for 197w.

Example

3 is in this sequence because (178*10^3 - 7)/9 = 19777 is prime.
Initial terms and associated primes:
a(1) = 0, 19;
a(2) = 1, 197;
a(3) = 3, 19777;
a(4) = 7; 197777777;
a(5) = 12, 19777777777777; etc.

Mathematica

Select[Range[0, 100000], PrimeQ[(178*10^# - 7)/9] &]

Crossrefs

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

Sequence in context: A045134 A215631 A081695 * A270122 A142958 A310214

Adjacent sequences: A290952 A290953 A290954 * A290956 A290957 A290958

Keyword

nonn,more,hard

Author

Robert Price, Aug 14 2017

Status

approved