A293593 - OEIS

%I #11 May 25 2024 19:37:44

%S 3,4,6,12,15,19,36,40,484,576,2328,2841,4008,4878,5190,10149,10383,

%T 11982,21082,46324,61156,83638,125886

%N Numbers k such that (26*10^k - 413)/9 is prime.

%C For k > 1, numbers k such that the digit 2 followed by k-2 occurrences of the digit 8 followed by the digits 43 is prime (see Example section).

%C a(24) > 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 28w43</a>.

%e 3 is in this sequence because (26*10^3 - 413)/9 = 2843 is prime.

%e Initial terms and associated primes:

%e a(1) = 3, 2843;

%e a(2) = 4, 28843;

%e a(3) = 6, 2888843;

%e a(4) = 12, 2888888888843;

%e a(5) = 15, 2888888888888843; etc.

%t Select[Range[2, 100000], PrimeQ[(26*10^# - 413)/9] &]

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

%K nonn,more,hard

%O 1,1

%A _Robert Price_, Oct 12 2017

%E a(23) from _Robert Price_, May 08 2018