A280016 Numbers k such that 8*10^k-99 is prime.

2, 3, 4, 9, 22, 50, 67, 82, 87, 89, 130, 134, 170, 179, 220, 832, 1425, 1648, 2356, 2434, 6556, 11391, 12610, 20278, 46509, 50253

Offset

1,1

Comments

For k>1, numbers such that the digit 7 followed by k-2 occurrences of the digit 9 followed by the digits 01 is prime (see Example section).

a(27) > 3*10^5.

Links

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

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

Makoto Kamada, Search for 79w01.

Example

3 is in this sequence because 8*10^3-99 = 7901 is prime.
Initial terms and primes associated:
a(1) = 2, 701;
a(2) = 3, 7901;
a(3) = 4, 79901;
a(4) = 9, 7999999901;
a(5) = 22, 79999999999999999999901; etc.

Mathematica

Select[Range[2, 100000], PrimeQ[8*10^#-99] &]

Crossrefs

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

Sequence in context: A352197 A192988 A361326 * A089243 A122534 A101135

Adjacent sequences: A280013 A280014 A280015 * A280017 A280018 A280019

Keyword

nonn,more,hard

Author

Robert Price, Feb 21 2017

Status

approved