A279467 Numbers k such that (14*10^k - 71) / 3 is prime.

1, 2, 3, 4, 6, 10, 14, 22, 25, 31, 43, 63, 123, 430, 508, 1457, 1701, 3371, 3429, 3548, 4582, 7463, 8852, 10594, 30621, 143662

Offset

1,2

Comments

For k > 1, numbers k such that the digit 4 followed by k-2 occurrences of the digit 6 followed by the digits 43 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 46w43.

Example

4 is in this sequence because (14*10^4 - 71) / 3 = 46643 is prime.
Initial terms and associated primes:
a(1) = 1, 23;
a(2) = 2, 443;
a(3) = 3, 4643;
a(4) = 4, 46643;
a(5) = 6, 4666643; etc.

Mathematica

Select[Range[1, 100000], PrimeQ[(14*10^# - 71) / 3] &]

Prog

(Magma) [n: n in [1..500] |IsPrime((14*10^n - 71) div 3)]; // Vincenzo Librandi Dec 14 2016
(PARI) is(n)=ispseudoprime((14*10^n - 71)/3) \\ Charles R Greathouse IV, Jun 13 2017

Crossrefs

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

Sequence in context: A256248 A089223 A240057 * A094861 A173473 A288807

Adjacent sequences: A279464 A279465 A279466 * A279468 A279469 A279470

Keyword

nonn,more,hard

Author

Robert Price, Dec 12 2016

Extensions

a(26) from Robert Price, Dec 28 2018

Status

approved