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%I #12 Jan 17 2019 13:44:10
%S 1,3,6,9,12,27,33,45,63,81,351,357,540,582,1998,2401,4140,4675,6963,
%T 7873,19749,138648,173826,190351
%N Numbers k such that (43*10^k + 101)/9 is prime.
%C For k > 1, numbers such that the digit 4 followed by k-2 occurrences of the digit 7 followed by the digits 89 is prime (see Example section).
%C a(25) > 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 47w89</a>
%e 3 is in this sequence because (43*10^3 + 101)/9 = 4789 is prime.
%e Initial terms and primes associated:
%e a(1) = 1, 59;
%e a(2) = 3, 4789;
%e a(3) = 6, 4777789;
%e a(4) = 9, 4777777789;
%e a(5) = 12, 4777777777789; etc.
%t Select[Range[1, 100000], PrimeQ[(43*10^# + 101)/9] &]
%Y Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K nonn,more,hard
%O 1,2
%A _Robert Price_, Nov 02 2017
%E a(22)-a(24) from _Robert Price_, Nov 30 2018
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