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%I #15 Feb 08 2021 21:55:53
%S 0,1,4,5,9,16,20,41,43,63,127,159,413,1591,1812,2031,2315,2437,4177,
%T 4860,5771,7060,7389,9925,34103,115879
%N Numbers k such that (94*10^k - 7) / 3 is prime.
%C For k > 0, numbers such that the digits 31 followed by k-1 occurrences of the digit 3 followed by the digit 1 is prime (see Example section).
%C a(27) > 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 313w1</a>.
%e 4 is in this sequence because (94*10^4 - 7) / 3 = 313331 is prime.
%e Initial terms and primes associated:
%e a(1) = 0, 29;
%e a(2) = 1, 311;
%e a(3) = 4, 313331;
%e a(4) = 5, 3133331;
%e a(5) = 9, 31333333331; etc.
%t Select[Range[0, 100000], PrimeQ[(94*10^# - 7) / 3] &]
%o (PARI) is(n)=ispseudoprime((94*10^n - 7)/3) \\ _Charles R Greathouse IV_, Jun 13 2017
%Y Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K nonn,more,hard
%O 1,3
%A _Robert Price_, Nov 18 2016
%E a(26) from _Robert Price_, Mar 10 2020
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