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%I #22 Jun 09 2024 18:06:13
%S 0,1,2,9,13,14,15,17,22,23,80,297,393,524,591,1107,1135,1179,1442,
%T 2819,3549,3756,3837,4903,5277,5639,7230,13147,14828,16158,18119,
%U 28880,99275,212339,254639
%N Numbers k such that 22*10^k + 7 is prime.
%C For k > 1, numbers k such that the digits 22 followed by k-1 occurrences of the digit 0 followed by the digit 7 is prime (see Example section).
%C a(36) > 3*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 220w7</a>.
%e 2 is in this sequence because 22*10^2+7 = 227 is prime.
%e Initial terms and associated primes:
%e a(1) = 0, 29;
%e a(2) = 1, 227;
%e a(3) = 2, 2207;
%e a(4) = 9, 22000000007;
%e a(5) = 13, 220000000000007, etc.
%t Select[Range[0, 100000], PrimeQ[22*10^# + 7] &]
%o (PARI) is(n)=ispseudoprime(22*10^n + 7) \\ _Charles R Greathouse IV_, Jun 13 2017
%Y Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K nonn,more
%O 1,3
%A _Robert Price_, Apr 11 2016
%E a(34)-a(35) from _Robert Price_, Jun 01 2023