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A274911 Numbers k such that 7*10^k + 87 is prime.

%I A274911 #29 Jun 09 2024 18:06:16
%S A274911 1,2,5,6,18,23,59,86,115,119,251,365,370,447,1672,3076,3973,5611,7687,
%T A274911 8824,13026,17141,17971,23346,29138,94373,94563,142189,156956,255167,
%U A274911 266731
%N A274911 Numbers k such that 7*10^k + 87 is prime.
%C A274911 For k > 1, numbers k such that the digit 7 followed by k-2 occurrences of the digit 0 followed by the digits 87 is prime (see Example section).
%C A274911 a(32) > 3*10^5.
%H A274911 Makoto Kamada, <a href="https://stdkmd.net/nrr">Factorization of near-repdigit-related numbers</a>.
%H A274911 Makoto Kamada, <a href="https://stdkmd.net/nrr/prime/prime_difficulty.txt">Search for 70w87</a>.
%e A274911 5 is in this sequence because 7*10^5 + 87 = 700087 is prime.
%e A274911 Initial terms and associated primes:
%e A274911 a(1) = 1, 157;
%e A274911 a(2) = 2, 787;
%e A274911 a(3) = 5, 700087;
%e A274911 a(4) = 6, 7000087;
%e A274911 a(5) = 18, 7000000000000000087, etc.
%t A274911 Select[Range[0, 100000], PrimeQ[7*10^# + 87] &]
%o A274911 (PARI) is(n)=ispseudoprime(7*10^n + 87) \\ _Charles R Greathouse IV_, Jun 13 2017
%Y A274911 Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K A274911 nonn,more
%O A274911 1,2
%A A274911 _Robert Price_, Nov 11 2016
%E A274911 a(28)-a(29) from _Robert Price_, Jul 27 2019
%E A274911 a(30)-a(31) from _Robert Price_, May 31 2023