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A293189 Numbers k such that (76*10^k - 211)/9 is prime. 0

%I #8 Aug 31 2019 10:05:56

%S 1,2,4,5,8,13,41,55,82,188,296,320,464,509,1036,1169,1808,2141,3842,

%T 12274,23141,76547,83816,85847,154013

%N Numbers k such that (76*10^k - 211)/9 is prime.

%C For k>1, numbers such that the digit 8 followed by k-2 occurrences of the digit 4 followed by the digits 21 is prime (see Example section).

%C a(26) > 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/primedifficulty.txt">Search for 84w21</a>.

%e 4 is in this sequence because (76*10^4 - 211)/9 = 84421 is prime.

%e Initial terms and primes associated:

%e a(1) = 1, 61;

%e a(2) = 2, 821;

%e a(3) = 4, 84421;

%e a(4) = 5, 844421;

%e a(5) = 8, 844444421; etc.

%t Select[Range[1, 100000], PrimeQ[(76*10^# - 211)/9] &]

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

%K nonn,more,hard

%O 1,2

%A _Robert Price_, Oct 01 2017

%E a(25) from _Robert Price_, Aug 31 2019

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Last modified March 28 18:04 EDT 2024. Contains 371254 sequences. (Running on oeis4.)