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%I #7 Jan 17 2019 13:44:08
%S 1,2,4,5,17,21,23,28,41,43,51,59,105,115,131,273,585,1519,2303,4791,
%T 4921,6019,7833,25711,27319,32497,37975,49381,87199
%N Numbers n such that (13*10^n + 197)/3 is prime.
%C For n>1, numbers such that the digit 4 followed by n-2 occurrences of the digit 3 followed by the digits 99 is prime (see the Example section).
%C a(30) > 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 43w99.</a>
%e 4 is in this sequence because (13*10^4 + 197)/3 = 43399 is prime.
%e Initial terms and primes associated:
%e a(1) = 1, 109;
%e a(2) = 2, 499;
%e a(3) = 4, 43399;
%e a(4) = 5, 433399;
%e a(5) = 17, 433333333333333399, etc.
%t Select[Range[0, 100000], PrimeQ[(13*10^# + 197)/3] &]
%o (PARI) is(n)=ispseudoprime((13*10^n+197)/3) \\ _Charles R Greathouse IV_, Jun 13 2017
%Y Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K nonn,more
%O 1,2
%A _Robert Price_, Aug 29 2016
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