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A274914 Numbers k such that 88*10^k + 7 is prime.

%I A274914 #18 May 25 2024 19:39:17
%S A274914 1,2,3,4,8,14,17,19,24,26,30,43,81,85,171,267,282,2384,4201,4450,6995,
%T A274914 7170,15049,15681,50547,67691,109022
%N A274914 Numbers k such that 88*10^k + 7 is prime.
%C A274914 For k > 0, numbers k such that the digits 88 followed by k-1 occurrences of the digit 0 followed by the digit 7 is prime (see Example section).
%C A274914 a(28) > 2*10^5.
%H A274914 Makoto Kamada, <a href="https://stdkmd.net/nrr">Factorization of near-repdigit-related numbers</a>.
%H A274914 Makoto Kamada, <a href="https://stdkmd.net/nrr/prime/prime_difficulty.txt">Search for 880w7</a>.
%e A274914 4 is in this sequence because 88*10^4 + 7 = 880007is prime.
%e A274914 Initial terms and associated primes:
%e A274914 a(1) = 1, 887;
%e A274914 a(2) = 2, 8807;
%e A274914 a(3) = 3, 88007;
%e A274914 a(4) = 4, 880007;
%e A274914 a(5) = 8, 8800000007, etc.
%t A274914 Select[Range[0, 100000], PrimeQ[88*10^# + 7] &]
%o A274914 (PARI) is(n)=ispseudoprime(88*10^n + 7) \\ _Charles R Greathouse IV_, Jun 13 2017
%Y A274914 Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K A274914 nonn,more
%O A274914 1,2
%A A274914 _Robert Price_, Nov 11 2016
%E A274914 a(27) from _Robert Price_, Mar 17 2020