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A274692 Numbers k such that 7*10^k + 43 is prime.

%I A274692 #17 May 02 2024 04:26:17
%S A274692 1,2,3,7,26,27,36,44,50,57,59,73,124,152,154,250,271,301,376,451,1177,
%T A274692 2299,3740,13159,14780,17435,30098,32521
%N A274692 Numbers k such that 7*10^k + 43 is prime.
%C A274692 For k > 1, numbers k such that the digit 7 followed by k-2 occurrences of the digit 0 followed by the digits 43 is prime (see Example section).
%C A274692 a(29) > 10^5.
%H A274692 Makoto Kamada, <a href="https://stdkmd.net/nrr">Factorization of near-repdigit-related numbers</a>.
%H A274692 Makoto Kamada, <a href="https://stdkmd.net/nrr/prime/prime_difficulty.txt">Search for 70w43</a>.
%e A274692 3 is in this sequence because 7*10^3 + 43 = 7043 is prime.
%e A274692 Initial terms and associated primes:
%e A274692 a(1) = 1, 113;
%e A274692 a(2) = 2, 743;
%e A274692 a(3) = 3, 7043;
%e A274692 a(4) = 7, 70000043;
%e A274692 a(5) = 26, 700000000000000000000000043, etc.
%t A274692 Select[Range[0, 100000], PrimeQ[7*10^# + 43] &]
%o A274692 (PARI) lista(nn) = for(n=1, nn, if(ispseudoprime(7*10^n + 43), print1(n, ", "))); \\ _Altug Alkan_, Jul 02 2016
%o A274692 (Magma) [n: n in [1..400] | IsPrime(7*10^n + 43)]; // _Vincenzo Librandi_, Jul 03 2016
%Y A274692 Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K A274692 nonn,more
%O A274692 1,2
%A A274692 _Robert Price_, Jul 02 2016