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A271646 Numbers k such that 22*10^k + 7 is prime.

%I A271646 #22 Jun 09 2024 18:06:13
%S A271646 0,1,2,9,13,14,15,17,22,23,80,297,393,524,591,1107,1135,1179,1442,
%T A271646 2819,3549,3756,3837,4903,5277,5639,7230,13147,14828,16158,18119,
%U A271646 28880,99275,212339,254639
%N A271646 Numbers k such that 22*10^k + 7 is prime.
%C A271646 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 A271646 a(36) > 3*10^5.
%H A271646 Makoto Kamada, <a href="https://stdkmd.net/nrr">Factorization of near-repdigit-related numbers</a>.
%H A271646 Makoto Kamada, <a href="https://stdkmd.net/nrr/prime/prime_difficulty.txt">Search for 220w7</a>.
%e A271646 2 is in this sequence because 22*10^2+7 = 227 is prime.
%e A271646 Initial terms and associated primes:
%e A271646 a(1) = 0, 29;
%e A271646 a(2) = 1, 227;
%e A271646 a(3) = 2, 2207;
%e A271646 a(4) = 9, 22000000007;
%e A271646 a(5) = 13, 220000000000007, etc.
%t A271646 Select[Range[0, 100000], PrimeQ[22*10^# + 7] &]
%o A271646 (PARI) is(n)=ispseudoprime(22*10^n + 7) \\ _Charles R Greathouse IV_, Jun 13 2017
%Y A271646 Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K A271646 nonn,more
%O A271646 1,3
%A A271646 _Robert Price_, Apr 11 2016
%E A271646 a(34)-a(35) from _Robert Price_, Jun 01 2023