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A270738 Numbers k such that 23*10^k - 7 is prime.

%I A270738 #24 May 26 2024 14:40:53
%S A270738 1,2,3,6,8,12,18,19,30,65,77,126,353,541,576,723,777,1024,1194,1507,
%T A270738 2379,2615,4008,4295,4495,4526,9996,10348,10673,14120,22350,70240,
%U A270738 93116,122070,136225,183710,224232,234025,270799
%N A270738 Numbers k such that 23*10^k - 7 is prime.
%C A270738 Numbers k such that the digits 22 followed by k-1 occurrences of the digit 9 followed by the digit 3 is prime (see Example section).
%C A270738 a(40) > 3*10^5.
%H A270738 Makoto Kamada, <a href="https://stdkmd.net/nrr/prime/prime_difficulty.txt">Search for 229w3</a>.
%e A270738 3 is in this sequence because 23*10^3-7 = 22993 is prime.
%e A270738 Initial terms and associated primes:
%e A270738 a(1) = 1, 223;
%e A270738 a(2) = 2, 2293;
%e A270738 a(3) = 3, 22993;
%e A270738 a(4) = 6, 22999993;
%e A270738 a(5) = 8, 2299999993, etc.
%t A270738 Select[Range[0, 100000], PrimeQ[23*10^# - 7] &] (* Corrected by _Georg Fischer_, Jul 22 2019 *)
%o A270738 (PARI) lista(nn) = {for(n=1, nn, if(ispseudoprime(23*10^n - 7), print1(n, ", "))); } \\ _Altug Alkan_, Mar 22 2016
%Y A270738 Cf. A056654, A268448, A269303, A270339, A270613.
%K A270738 nonn,more
%O A270738 1,2
%A A270738 _Robert Price_, Mar 22 2016
%E A270738 a(34)-a(36) from _Robert Price_, Feb 25 2020
%E A270738 a(37)-a(39) from _Robert Price_, May 31 2023