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%I #8 Jan 17 2019 13:44:09
%S 1,2,4,8,12,15,21,31,48,151,723,879,1811,3444,5104,5512,5695,6723,
%T 9082,14944,15184,18512,35141,81985,123563,155203,165134,165239
%N Numbers k such that (7*10^k + 107)/3 is prime.
%C For k>1, numbers such that the digit 2 followed by k-2 occurrences of the digit 3 followed by the digits 69 is prime (see Example section).
%C a(29) > 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 23w69.</a>
%e 4 is in this sequence because (7*10^4 + 107)/3 = 23369 is prime.
%e Initial terms and primes associated:
%e a(1) = 1, 59;
%e a(2) = 2, 269;
%e a(3) = 4, 23369;
%e a(4) = 8, 233333369;
%e a(5) = 12, 2333333333369; etc.
%t Select[Range[0, 100000], PrimeQ[(7*10^# + 107)/3] &]
%Y Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K nonn,more,hard
%O 1,2
%A _Robert Price_, Feb 20 2017
%E a(25)-a(28) from _Robert Price_, Feb 21 2018
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