A281828 Numbers k such that 7*10^k - 89 is prime.

3, 4, 6, 7, 12, 13, 23, 28, 42, 51, 66, 247, 291, 623, 3429, 3892, 4416, 7366, 7991, 8737, 10151, 10209, 40129, 41604, 50818, 68088

Offset

1,1

Comments

For k > 1, numbers k such that the digit 6 followed by k-2 occurrences of the digit 9 followed by the digits 11 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 69w11.

Example

4 is in this sequence because 7*10^4 - 89 = 69911 is prime.
Initial terms and associated primes:
a(1) = 3, 6911;
a(2) = 4, 69911;
a(3) = 6, 6999911;
a(4) = 7, 69999911;
a(5) = 12, 6999999999911; etc.

Mathematica

Select[Range[2, 100000], PrimeQ[7*10^# - 89] &]

Crossrefs

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

Sequence in context: A033162 A105133 A211384 * A002982 A290432 A276783

Adjacent sequences: A281825 A281826 A281827 * A281829 A281830 A281831

Keyword

nonn,more,hard

Author

Robert Price, Jan 31 2017

Status

approved