A284427 Numbers k such that (206*10^k - 17)/9 is prime.

1, 2, 4, 5, 11, 16, 23, 193, 217, 412, 413, 2662, 4388, 5383, 6391, 7847, 12698, 14369, 16358, 20345, 28483, 75073, 79268, 100189, 116687

Offset

1,2

Comments

For k > 1, numbers k such that the digits 22 followed by k-1 occurrences of the digit 8 followed by the digit 7 is prime (see Example section).

a(26) > 2*10^5.

Links

Table of n, a(n) for n=1..25.

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

Makoto Kamada, Search for 228w7.

Example

4 is in this sequence because (206*10^4-17)/9 = 228887 is prime.
Initial terms and associated primes:
a(1) = 1, 227;
a(2) = 2, 2287;
a(3) = 4, 228887;
a(4) = 5, 2288887;
a(5) = 11, 2288888888887; etc.

Mathematica

Select[Range[0, 100000], PrimeQ[(206*10^#-17)/9] &]

Crossrefs

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

Sequence in context: A191289 A370633 A256570 * A366702 A089416 A056817

Adjacent sequences: A284424 A284425 A284426 * A284428 A284429 A284430

Keyword

nonn,more,hard

Author

Robert Price, Mar 26 2017

Extensions

a(24)-a(25) from Robert Price, Jan 14 2020

Status

approved