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A271269 Numbers k such that 8*10^k - 49 is prime.

%I A271269 #24 Jun 09 2024 18:06:10
%S A271269 1,2,3,8,24,49,57,74,104,131,144,162,182,246,302,352,557,581,589,704,
%T A271269 939,1181,1937,2157,4463,6013,7266,8504,8691,16129,20108,40677,74234,
%U A271269 112018
%N A271269 Numbers k such that 8*10^k - 49 is prime.
%C A271269 For k > 1, numbers k such that the digit 7 followed by k-2 occurrences of the digit 9 followed by the digits 51 is prime (see Example section).
%C A271269 a(35) > 2*10^5.
%H A271269 Makoto Kamada, <a href="https://stdkmd.net/nrr/prime/prime_difficulty.txt">Search for 79w51</a>.
%e A271269 3 is in this sequence because 8*10^3 - 49 = 7951 is prime.
%e A271269 Initial terms and associated primes:
%e A271269 a(1) = 1, 31;
%e A271269 a(2) = 2, 751;
%e A271269 a(3) = 3, 7951;
%e A271269 a(4) = 8, 799999951;
%e A271269 a(6) = 24, 7999999999999999999999951, etc.
%t A271269 Select[Range[0, 100000], PrimeQ[8*10^# - 49] &]
%o A271269 (PARI) is(n)=isprime(8*10^n - 49) \\ _Charles R Greathouse IV_, Feb 16 2017
%Y A271269 Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929.
%K A271269 nonn,more
%O A271269 1,2
%A A271269 _Robert Price_, Apr 03 2016
%E A271269 a(34) from _Robert Price_, Aug 20 2019