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%I #11 Jan 17 2019 13:44:09
%S 2,3,15,17,21,38,93,323,410,413,444,884,1836,2192,3527,4700,6260,6944,
%T 8229,10320,27480,31425,32937,108545,155024,182177,196965
%N Numbers k such that (14*10^k - 221)/9 is prime.
%C For k>1, numbers such that the digit 1 followed by k-2 occurrences of the digit 5 followed by the digits 31 is prime (see Example section).
%C a(28) > 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 15w31.</a>
%e 3 is in this sequence because (14*10^3 - 221)/9 = 1531 is prime.
%e Initial terms and primes associated:
%e a(1) = 2, 131;
%e a(2) = 3, 1531;
%e a(3) = 15, 1555555555555531;
%e a(4) = 17, 155555555555555531;
%e a(5) = 21, 1555555555555555555531; etc.
%t Select[Range[2, 100000], PrimeQ[(14*10^# - 221)/9] &]
%o (PARI) isok(k) = ispseudoprime((14*10^k - 221)/9); \\ _Altug Alkan_, Apr 12 2018
%Y Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K nonn,more,hard
%O 1,1
%A _Robert Price_, Feb 13 2017
%E a(24)-a(27) from _Robert Price_, Apr 12 2018
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