A288824 Numbers k such that (26*10^k + 61)/3 is prime.

0, 1, 2, 8, 10, 17, 18, 30, 37, 42, 47, 100, 286, 287, 440, 1074, 1124, 2298, 2879, 4948, 4950, 12730, 17429, 31768, 47726, 64220, 80200, 127240

Offset

1,3

Comments

For k > 1, numbers k such that the digit 8 followed by k-2 occurrences of the digit 6 followed by the digits 87 is prime (see Example section).

a(29) > 2*10^5.

Links

Table of n, a(n) for n=1..28.

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

Makoto Kamada, Search for 86w87.

Example

2 is in this sequence because (26*10^2 + 61)/3 = 887 is prime.
Initial terms and associated primes:
a(1) = 0, 29;
a(2) = 1, 107;
a(3) = 2, 887;
a(4) = 8, 866666687;
a(5) = 10, 86666666687; etc.

Mathematica

Select[Range[0, 100000], PrimeQ[(26*10^# + 61)/3] &]

Crossrefs

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

Sequence in context: A038638 A226831 A197115 * A190042 A297293 A294157

Adjacent sequences: A288821 A288822 A288823 * A288825 A288826 A288827

Keyword

nonn,more,hard

Author

Robert Price, Jun 17 2017

Extensions

a(28) from Robert Price, Oct 13 2019

Status

approved