A281839 - OEIS

%I #14 Jun 03 2024 23:31:21

%S 1,2,3,4,7,10,17,22,32,33,39,70,79,97,273,304,905,2474,3523,10348,

%T 15155,22252,22966,70858,82504,90793

%N Numbers k such that 5*10^k + 21 is prime.

%C For k > 1, numbers k such that the digit 5 followed by k-2 occurrences of the digit 0 followed by the digits 21 is prime (see Example section).

%C a(27) > 2*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 50w21</a>.

%e 4 is in this sequence because 5*10^4 + 21 = 50021 is prime.

%e Initial terms and associated primes:

%e a(1) = 1, 71;

%e a(2) = 2, 521;

%e a(3) = 3, 5021;

%e a(4) = 4, 50021;

%e a(5) = 7, 50000021; etc.

%p select(k->isprime(5*10^k+21),[$1..1000]); # _Muniru A Asiru_, Jan 05 2019

%t Select[Range[0, 100000], PrimeQ[5*10^# + 21] &]

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

%K nonn,more,hard

%O 1,2

%A _Robert Price_, Jan 31 2017