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%I #15 Jan 17 2019 13:44:08
%S 0,1,2,5,7,8,12,42,80,91,138,169,1376,1715,3212,6410,8492,9257,10584,
%T 11123,13819,17532,20369,22513,56156,69626,71498,83201,155022,197778
%N Numbers k such that (4*10^k + 173)/3 is prime.
%C For k>1, numbers such that the digit 1 followed by k-2 occurrences of the digit 3 followed by the digits 91 is prime (see Example section).
%C a(31) > 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 13w91.</a>
%e 2 is in this sequence because (4*10^2 + 173) / 3 = 191 is prime.
%e Initial terms and primes associated:
%e a(1) = 0, 59;
%e a(2) = 1, 71;
%e a(3) = 2, 191;
%e a(4) = 5, 133391;
%e a(5) = 7, 13333391; etc.
%t Select[Range[0, 100000], PrimeQ[(4*10^# + 173) / 3] &]
%Y Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K nonn,more,hard
%O 1,3
%A _Robert Price_, Jan 09 2017
%E a(29)-a(30) from _Robert Price_, Feb 16 2018
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