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A270831 Numbers k such that (7*10^k + 71)/3 is prime.

%I A270831 #15 Jun 02 2024 09:28:58
%S A270831 1,2,3,4,5,7,23,29,37,39,40,89,115,189,227,253,301,449,533,607,969,
%T A270831 1036,1207,1407,1701,3493,7147,11342,21638,22327,25575,25648,34079,
%U A270831 39974,47719,49913,74729,100737,103531,168067
%N A270831 Numbers k such that (7*10^k + 71)/3 is prime.
%C A270831 For k > 1, numbers k such that the digit 2 followed by k-2 occurrences of the digit 3 followed by the digits 57 is prime (see Example section).
%C A270831 a(41) > 2*10^5.
%H A270831 Makoto Kamada, <a href="https://stdkmd.net/nrr/prime/prime_difficulty.txt">Search for 23w57</a>.
%e A270831 3 is in this sequence because (7*10^3 + 71)/3 = 2357 is prime.
%e A270831 Initial terms and associated primes:
%e A270831 a(1) = 1, 47;
%e A270831 a(2) = 2, 257;
%e A270831 a(3) = 3, 2357;
%e A270831 a(4) = 4, 23357;
%e A270831 a(5) = 5, 233357, etc.
%t A270831 Select[Range[0, 100000], PrimeQ[(7*10^# + 71)/3] &]
%o A270831 (PARI) lista(nn) = {for(n=1, nn, if(ispseudoprime((7*10^n + 71)/3), print1(n, ", "))); } \\ _Altug Alkan_, Mar 23 2016
%Y A270831 Cf. A056654, A268448, A269303, A270339, A270613.
%K A270831 nonn
%O A270831 1,2
%A A270831 _Robert Price_, Mar 23 2016
%E A270831 a(38)-a(40) from _Robert Price_, May 21 2018