A281276 Numbers k such that (2*10^k + 457)/9 is prime.

1, 2, 4, 5, 7, 8, 11, 13, 20, 22, 38, 47, 76, 269, 299, 527, 724, 1214, 2243, 2327, 12635, 15064, 28921, 34303, 49117, 58151, 89279, 199283

Offset

1,2

Comments

For k>2, numbers such that k-2 occurrences of the digit 2 followed by the digits 73 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 2w73.

Example

2 is in this sequence because (2*10^2 + 457) / 9 = 73 is prime.
Initial terms and primes associated:
a(1) = 1, 53;
a(2) = 2, 73;
a(3) = 4, 2273;
a(4) = 5, 22273;
a(5) = 7, 2222273; etc.

Mathematica

Select[Range[0, 100000], PrimeQ[(2*10^# + 457) / 9] &]

Crossrefs

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

Sequence in context: A001606 A014554 A114147 * A025516 A191174 A261188

Adjacent sequences: A281273 A281274 A281275 * A281277 A281278 A281279

Keyword

nonn,more,hard

Author

Robert Price, Jan 18 2017

Extensions

a(28) from Robert Price, Jan 08 2018

Status

approved