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%I #14 May 11 2024 19:24:41
%S 2,3,5,6,8,9,18,41,209,815,2573,3590,4439,4568,6720,16491,20069,22406,
%T 36756,43076,53465,58550,70896,74858,184950
%N Numbers k such that (14*10^k + 211)/9 is prime.
%C For k > 1, numbers k such that the digit 1 followed by k-2 occurrences of the digit 5 followed by the digits 79 is prime (see Example section).
%C a(26) > 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 15w79</a>.
%e 2 is in this sequence because (14*10^2 + 211)/9 = 179 s prime.
%e Initial terms and associated primes:
%e a(1) = 2, 179;
%e a(2) = 3, 1579;
%e a(3) = 5, 155579;
%e a(4) = 6, 1555579;
%e a(5) = 8, 155555579; etc.
%t Select[Range[0 100000], PrimeQ[(14*10^# + 211)/9] &]
%o (PARI) isok(k) = isprime((14*10^k + 211)/9); \\ _Michel Marcus_, Nov 12 2017
%Y Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K nonn,more,hard
%O 1,1
%A _Robert Price_, Nov 11 2017
%E a(25) from _Robert Price_, Apr 21 2018