A273097 - OEIS

%I #22 May 26 2024 14:28:42

%S 1,2,4,5,13,25,27,32,37,38,40,45,57,80,91,151,214,441,644,764,797,

%T 1222,2329,2931,4324,21794,22396,24041,46420,51489,55165,126625

%N Numbers k such that 4*10^k + 87 is prime.

%C For k > 1, numbers k such that the digit 4 followed by k-2 occurrences of the digit 0 followed by the digits 87 is prime (see Example section).

%C a(33) > 3*10^5.

%H Makoto Kamada, <a href="https://stdkmd.net/nrr">Factorization of near-repdigit-related numbers</a>.

%H Makoto Kamada, <a href="https://stdkmd.net/nrr/prime/prime_difficulty.txt">Search for 40w87</a>.

%e 4 is in this sequence because 4*10^4 + 87 = 40087 is prime.

%e Initial terms and associated primes:

%e a(1) = 1, 127:

%e a(2) = 2, 487;

%e a(3) = 4, 40087;

%e a(4) = 5, 400087;

%e a(5) = 13, 40000000000087, etc.

%t Select[Range[0, 100000], PrimeQ[4*10^# + 87] &]

%o (PARI) is(n)=ispseudoprime(4*10^n + 87) \\ _Charles R Greathouse IV_, Jun 13 2017

%Y Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.

%K nonn,more

%O 1,2

%A _Robert Price_, May 15 2016

%E a(32) from _Robert Price_, Aug 15 2018