A288823 Numbers k such that (86*10^k - 221)/9 is prime.

1, 4, 7, 13, 17, 31, 47, 64, 226, 251, 268, 310, 352, 394, 478, 599, 1529, 1679, 11590, 12922, 13151, 18808, 47188, 52450, 83038, 93217, 128086, 154853, 175774

Offset

1,2

Comments

For k > 1, numbers k such that the digit 9 followed by k-2 occurrences of the digit 5 followed by the digits 31 is prime (see Example section).

a(39) > 2*10^5.

Links

Table of n, a(n) for n=1..29.

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

Makoto Kamada, Search for 95w31.

Example

4 is in this sequence because (86*10^4 - 221)/9 = 95531 is prime.
Initial terms and associated primes:
a(1) = 1, 71;
a(2) = 4, 95531;
a(3) = 7, 95555531;
a(4) = 13, 95555555555531;
a(5) = 17, 955555555555555531; etc.

Mathematica

Select[Range[1, 100000], PrimeQ[(86*10^# - 221)/9] &]

Crossrefs

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

Sequence in context: A265704 A110267 A049698 * A310823 A074136 A310824

Adjacent sequences: A288820 A288821 A288822 * A288824 A288825 A288826

Keyword

nonn,more,hard

Author

Robert Price, Jun 17 2017

Extensions

a(27)-a(29) from Robert Price, Aug 31 2019

Status

approved