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 A282457 Numbers k such that 7*10^k - 47 is prime. 0

%I

%S 1,2,5,8,11,14,15,30,31,42,69,85,120,128,449,685,686,741,2165,9110,

%T 9150,11366,13154,1291,1521,33321,40758,50789,51440,65492,82509,93887

%N Numbers k such that 7*10^k - 47 is prime.

%C For k>1, numbers such that the digit 6 followed by k-2 occurrences of the digit 9 followed by the digits 53 is prime (see Example section).

%C a(33) > 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 69w53</a>.

%e 2 is in this sequence because 7*10^2 - 47 = 653 is prime.

%e Initial terms and primes associated:

%e a(1) = 1, 23;

%e a(2) = 2, 653;

%e a(3) = 5, 699953;

%e a(4) = 8, 699999953;

%e a(5) = 11, 699999999953; etc.

%t Select[Range[1, 100000], PrimeQ[7*10^# - 47] &]

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

%K nonn,more,hard

%O 1,2

%A _Robert Price_, Feb 15 2017

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Last modified April 1 01:51 EDT 2020. Contains 333153 sequences. (Running on oeis4.)