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%I #24 Jan 17 2019 13:44:09
%S 1,3,4,6,7,15,18,57,75,82,358,1347,2278,2577,4156,6712,7471,14437,
%T 15247,28251,62914,78213,158694
%N Numbers k such that (13*10^k + 401)/9 is prime.
%C For k>1, numbers such that the digit 1 followed by k-2 occurrences of the digit 4 followed by the digits 89 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/primedifficulty.txt">Search for 14w89</a>
%e 3 is in this sequence because (13*10^3 + 401)/9 = 1489 is prime.
%e Initial terms and primes associated:
%e a(1) = 1, 59;
%e a(2) = 3, 1489;
%e a(3) = 4, 14489;
%e a(4) = 6, 1444489;
%e a(5) = 7, 14444489; etc.
%t Select[Range[0, 100000], PrimeQ[(13*10^# + 401)/9] &]
%Y Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K nonn,more,hard
%O 1,2
%A _Robert Price_, Oct 09 2017
%E a(23) from _Robert Price_, Jan 08 2018
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