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%I #17 May 26 2024 14:29:00
%S 2,3,5,6,8,11,15,25,38,54,101,105,158,295,303,389,482,558,693,1965,
%T 3503,9818,13255,30351,51285,54767,57384,88808,109645,174301,187062
%N Numbers k such that (2*10^k - 71)/3 is prime.
%C For k > 1, numbers k such that the number consisting of k-2 occurrences of the digit 6 followed by the digits 43 is prime (see Example section).
%C a(32) > 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 6w43</a>.
%e 5 is in this sequence because (2*10^5 - 71) / 3 = 66643 is prime.
%e Initial terms and associated primes:
%e a(1) = 2, 43;
%e a(2) = 3, 643;
%e a(3) = 5, 66643;
%e a(4) = 6, 666643;
%e a(5) = 8, 66666643; etc.
%t Select[Range[2, 100000], PrimeQ[(2*10^# - 71) / 3] &]
%o (PARI) is(n)=isprime((2*10^n - 71)/3) \\ _Charles R Greathouse IV_, Jan 03 2017
%Y Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K nonn,more,hard
%O 1,1
%A _Robert Price_, Jan 03 2017
%E a(29) - a(31) from _Robert Price_, Dec 31 2017
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