A281839 Numbers k such that 5*10^k + 21 is prime.

1, 2, 3, 4, 7, 10, 17, 22, 32, 33, 39, 70, 79, 97, 273, 304, 905, 2474, 3523, 10348, 15155, 22252, 22966, 70858, 82504, 90793

Offset

1,2

Comments

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

a(27) > 2*10^5.

Links

Table of n, a(n) for n=1..26.

Makoto Kamada, Factorization of near-repdigit-related numbers.

Makoto Kamada, Search for 50w21.

Example

4 is in this sequence because 5*10^4 + 21 = 50021 is prime.
Initial terms and primes associated:
a(1) = 1, 71;
a(2) = 2, 521;
a(3) = 3, 5021;
a(4) = 4, 50021;
a(5) = 7, 50000021; etc.

Maple

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

Mathematica

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

Crossrefs

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

Sequence in context: A235648 A051449 A018143 * A136570 A082766 A119016

Adjacent sequences: A281836 A281837 A281838 * A281840 A281841 A281842

Keyword

nonn,more,hard

Author

Robert Price, Jan 31 2017

Status

approved