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%I #12 Jan 17 2019 13:44:09
%S 1,4,6,10,16,37,60,64,78,96,166,256,1294,1398,2044,2244,5080,7464,
%T 8041,17929,18144,29080,32623
%N Numbers k such that (19*10^k - 37)/9 is prime.
%C For k>1, numbers such that the digit 2 followed by k-2 occurrences of the digit 1 followed by the digits 07 is prime (see Example section).
%C a(24) > 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 21w07.</a>
%e 4 is in this sequence because (19*10^4 - 37)/9 = 21107 is prime.
%e Initial terms and primes associated:
%e a(1) = 1, 17;
%e a(2) = 4, 21107;
%e a(3) = 6, 2111107;
%e a(4) = 10, 21111111107;
%e a(5) = 16, 21111111111111107; etc.
%t Select[Range[1, 100000], PrimeQ[(19*10^# - 37)/9] &]
%o (PARI) isok(n) = isprime((19*10^n - 37)/9); \\ _Indranil Ghosh_, Mar 09 2017
%Y Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K nonn,more,hard
%O 1,2
%A _Robert Price_, Mar 08 2017
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