A286435 - OEIS

%I #12 May 25 2024 14:28:21

%S 2,3,6,9,27,48,60,74,161,242,269,518,1092,1220,1883,2618,3266,6699,

%T 8855,17022,17454,101222

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

%C 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).

%C a(23) > 2*10^5.

%H Makoto Kamada, <a href="https://stdkmd.net/nrr">Factorization of near-repdigit-related numbers</a>.

%H Makoto Kamada, <a href="https://stdkmd.net/nrr/prime/prime_difficulty.txt">Search for 62w17</a>.

%e 3 is in this sequence because (56*10^3 - 47)/9 = 6217 is prime.

%e Initial terms and associated primes:

%e a(1) = 2, 617;

%e a(2) = 3, 6217;

%e a(3) = 6, 6222217;

%e a(4) = 9, 6222222217;

%e a(5) = 27, 6222222222222222222222222217; etc.

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

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

%K nonn,more,hard

%O 1,1

%A _Robert Price_, May 09 2017

%E a(22) from _Robert Price_, Feb 25 2019