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%I #15 May 18 2024 11:34:07
%S 3,4,6,11,12,17,21,54,82,112,189,235,426,442,4897,12463,14383,18630,
%T 38669,95752,108238,179265
%N Numbers k such that 4*10^k - 71 is prime.
%C For k > 1, numbers k such that the digit 3 followed by k-2 occurrences of the digit 9 followed by the digits 29 is prime (see Example section).
%C a(23) > 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/prime_difficulty.txt">Search for 39w29</a>.
%e 3 is in this sequence because 4*10^3 - 71 = 3929 is prime.
%e Initial terms and associated primes:
%e a(1) = 3, 3929;
%e a(2) = 4, 39929;
%e a(3) = 6, 3999929;
%e a(4) = 11, 399999999929;
%e a(5) = 12, 3999999999929; etc.
%t Select[Range[2, 100000], PrimeQ[4*10^# - 71] &]
%Y Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K nonn,more,hard
%O 1,1
%A _Robert Price_, Nov 10 2017
%E a(21)-a(22) from _Robert Price_, Dec 28 2018