A285940 Numbers k such that (2*10^k + 67)/3 is prime.

0, 1, 2, 4, 7, 40, 43, 74, 98, 181, 186, 428, 532, 644, 1664, 2016, 2476, 2911, 2959, 3007, 5964, 7758, 22231, 92152

Offset

1,3

Comments

For k>1, numbers such that k-2 occurrences of the digit 6 followed by the digits 89 is prime (see Example section).

a(25) > 2*10^5.

Links

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

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

Makoto Kamada, Search for 6w89.

Example

4 is in this sequence because (2*10^4 + 67)/3 = 6689 is prime.
Initial terms and primes associated:
a(1) = 0, 23;
a(2) = 1, 29;
a(3) = 2, 89;
a(4) = 4, 6689;
a(5) = 7, 6666689; etc.

Mathematica

Select[Range[0, 100000], PrimeQ[(2*10^# + 67)/3] &]

Crossrefs

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

Sequence in context: A295125 A082537 A004577 * A211186 A302938 A076719

Adjacent sequences: A285937 A285938 A285939 * A285941 A285942 A285943

Keyword

nonn,more,hard

Author

Robert Price, Apr 29 2017

Status

approved