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%I #13 Mar 04 2019 20:18:46
%S 5,6,18,27,48,51,92,396,678,1259,2085,2820,3009,4311,5015,7775,7955,
%T 8595,120380,166721
%N Numbers k such that (49*10^k - 139)/9 is prime.
%C For k > 1, numbers such that the digit 5 followed by k-2 occurrences of the digit 4 followed by the digits 29 is prime (see Example section).
%C a(21) > 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 54w29</a>
%e 5 is in this sequence because (49*10^5 - 139)/9 = 544429 is prime.
%e Initial terms and primes associated:
%e a(1) = 5, 544429;
%e a(2) = 6, 5444429;
%e a(3) = 18, 5444444444444444429;
%e a(4) = 27, 5444444444444444444444444429; etc.
%t Select[Range[1, 100000], PrimeQ[(49*10^# - 139)/9] &]
%o (PARI) isok(k) = isprime((49*10^k - 139)/9); \\ _Michel Marcus_, Dec 01 2017
%Y Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K nonn,more,hard
%O 1,1
%A _Robert Price_, Nov 30 2017
%E a(19)-a(20) from _Robert Price_, Mar 04 2019
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