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%I #9 Mar 01 2020 19:39:37
%S 5,9,17,47,54,75,191,207,267,894,2099,7164,8625,10865,20394,22251,
%T 23088,29015,92369
%N Numbers k such that (397*10^k + 53)/9 is prime.
%C For k > 1, numbers such that the digits 44 followed by k-1 occurrences of the digit 1 followed by the digit 7 is prime (see Example section).
%C a(20) > 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 441w7</a>.
%e 5 is in this sequence because (397*10^5 + 531)/9 = 4411117 is prime.
%e Initial terms and primes associated:
%e a(1) = 5, 4411117;
%e a(2) = 9, 44111111117;
%e a(3) = 17, 4411111111111111117; etc.
%t Select[Range[0, 100000], PrimeQ[(397*10^# + 53)/9] &]
%Y Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K nonn,more,hard
%O 1,1
%A _Robert Price_, Nov 24 2017
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