A286435 Numbers k such that (56*10^k - 47)/9 is prime.

2, 3, 6, 9, 27, 48, 60, 74, 161, 242, 269, 518, 1092, 1220, 1883, 2618, 3266, 6699, 8855, 17022, 17454, 101222

Offset

1,1

Comments

For k > 1, numbers k such that the digit 6 followed by k-2 occurrences of the digit 2 followed by the digits 17 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 62w17.

Example

3 is in this sequence because (56*10^3 - 47)/9 = 6217 is prime.
Initial terms and associated primes:
a(1) = 2, 617;
a(2) = 3, 6217;
a(3) = 6, 6222217;
a(4) = 9, 6222222217;
a(5) = 27, 6222222222222222222222222217; etc.

Mathematica

Select[Range[0, 100000], PrimeQ[(56*10^# - 47)/9] &]

Crossrefs

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

Sequence in context: A133385 A349148 A002076 * A145761 A071714 A077753

Adjacent sequences: A286432 A286433 A286434 * A286436 A286437 A286438

Keyword

nonn,more,hard

Author

Robert Price, May 09 2017

Extensions

a(22) from Robert Price, Feb 25 2019

Status

approved