A287680 Numbers k such that 8*10^k + 81 is prime.

0, 2, 3, 9, 11, 26, 39, 42, 58, 75, 105, 167, 279, 561, 2152, 2378, 3774, 5001, 5736, 9470, 13419, 14535, 32291, 89800, 146421

Offset

1,2

Comments

For k > 1, numbers k such that the digit 8 followed by k-2 occurrences of the digit 0 followed by the digits 81 is prime (see Example section).

a(26) > 2*10^5.

Links

Table of n, a(n) for n=1..25.

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

Makoto Kamada, Search for 80w81.

Example

3 is in this sequence because 8*10^3 + 81 = 8081 is prime.
Initial terms and associated primes:
a(1) = 0, 89;
a(2) = 2, 881;
a(3) = 3, 8081;
a(4) = 9, 8000000081;
a(5) = 11, 800000000081; etc.

Mathematica

Select[Range[0, 100000], PrimeQ[(8*10^# + 81] &]

Crossrefs

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

Sequence in context: A372073 A089645 A214259 * A242680 A275767 A088086

Adjacent sequences: A287677 A287678 A287679 * A287681 A287682 A287683

Keyword

nonn,more,hard,changed

Author

Robert Price, May 29 2017

Extensions

a(25) from Robert Price, Aug 22 2019

Status

approved