A293824 Numbers k such that 28*10^k + 1 is prime.

0, 1, 2, 3, 4, 5, 16, 19, 22, 37, 58, 547, 570, 817, 1068, 1909, 12809, 14470, 18209, 33176, 33412, 42176, 49798, 89401

Offset

1,3

Comments

For k > 0, numbers k such that the digits 28 followed by k-1 occurrences of the digit 0 followed by the digit 1 is prime (see Example section).

a(25) > 10^5.

Links

Table of n, a(n) for n=1..24.

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

Makoto Kamada, Search for 280w1.

Example

2 is in this sequence because 28*10^2 + 1 = 2801 is prime.
Initial terms and associated primes:
a(1) = 0, 29;
a(2) = 1, 281;
a(3) = 2, 2801;
a(4) = 3, 280001;
a(5) = 4, 2800001; etc.

Mathematica

Select[Range[0, 100000], PrimeQ[28*10^# + 1] &]

Prog

(PARI) lista(nn) = for(n = 0, nn, if(isprime(28*10^n+1), print1(n, ", "))) \\ Iain Fox, Oct 18 2017

Crossrefs

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

Sequence in context: A004834 A075687 A283526 * A095181 A240906 A117885

Adjacent sequences: A293821 A293822 A293823 * A293825 A293826 A293827

Keyword

nonn,more,hard

Author

Robert Price, Oct 16 2017

Status

approved