A281296 Numbers k such that (68*10^k + 1)/3 is prime.

0, 1, 2, 8, 9, 14, 15, 20, 32, 37, 123, 134, 135, 288, 361, 414, 469, 510, 6272, 11204, 11694, 16788, 20718

Offset

1,3

Comments

For k > 0, numbers k such that the digits 22 followed by k-1 occurrences of the digit 6 followed by the digit 7 is prime (see Example section).

a(24) > 2*10^5.

Links

Table of n, a(n) for n=1..23.

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

Makoto Kamada, Search for 226w7.

Example

2 is in this sequence because (68*10^2 + 1)/3 = 2267 is prime.
Initial terms and associated primes:
a(1) = 0, 23;
a(2) = 1, 227;
a(3) = 2, 2267;
a(4) = 8, 2266666667;
a(5) = 9, 22666666667; etc.

Mathematica

Select[Range[0, 100000], PrimeQ[(68*10^# + 1)/3] &]

Crossrefs

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

Sequence in context: A365517 A166686 A064833 * A073606 A305177 A327299

Adjacent sequences: A281293 A281294 A281295 * A281297 A281298 A281299

Keyword

nonn,more,hard

Author

Robert Price, Jan 19 2017

Status

approved