cp's OEIS Frontend

A274456 Numbers k such that 5*10^k + 77 is prime.

%I A274456 #16 May 02 2024 04:26:44
%S A274456 1,2,3,4,6,8,19,27,37,56,66,136,148,387,534,536,1273,1593,1796,2026,
%T A274456 2164,2502,6128,18714,23327,25427,46461,88182,88377,104326,127153,
%U A274456 135019
%N A274456 Numbers k such that 5*10^k + 77 is prime.
%C A274456 For k > 1, numbers k such that the digit 5 followed by k-2 occurrences of the digit 0 followed by the digits 77 is prime (see Example section).
%C A274456 a(33) > 2*10^5.
%H A274456 Makoto Kamada, <a href="https://stdkmd.net/nrr">Factorization of near-repdigit-related numbers</a>.
%H A274456 Makoto Kamada, <a href="https://stdkmd.net/nrr/prime/prime_difficulty.txt">Search for 50w77</a>.
%e A274456 3 is in this sequence because 5*10^3 + 77 = 5077 is prime.
%e A274456 Initial terms and associated primes:
%e A274456 a(1) = 1, 127;
%e A274456 a(2) = 2, 577;
%e A274456 a(3) = 3, 5077;
%e A274456 a(4) = 4, 50077;
%e A274456 a(5) = 6, 5000077, etc.
%t A274456 Select[Range[0, 100000], PrimeQ[5*10^# + 77] &]
%o A274456 (PARI) is(n)=ispseudoprime(5*10^n + 77) \\ _Charles R Greathouse IV_, Jun 13 2017
%Y A274456 Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K A274456 nonn,more
%O A274456 1,2
%A A274456 _Robert Price_, Jun 23 2016
%E A274456 a(30)-a(32) from _Robert Price_, Dec 30 2018