A281512 Numbers k such that 5*10^k - 81 is prime.

2, 3, 4, 13, 15, 22, 24, 28, 111, 148, 248, 294, 476, 1017, 1875, 2061, 2068, 2593, 4462, 6078, 14457, 14527, 23679, 33356, 36484, 50890, 51608, 113224

Offset

1,1

Comments

For k > 1, numbers such that the digit 4 followed by k-2 occurrences of the digit 9 followed by the digits 19 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 49w19.

Example

3 is in this sequence because 5*10^3 - 81 = 4919 is prime.
Initial terms and associated primes:
a(1) = 2, 419;
a(2) = 3, 4919;
a(3) = 4, 49919;
a(4) = 13, 49999999999919;
a(5) = 15, 4999999999999919; etc.

Mathematica

Select[Range[1, 100000], PrimeQ[5*10^# - 81] &]

Crossrefs

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

Sequence in context: A338478 A126129 A102947 * A295396 A236440 A357954

Adjacent sequences: A281509 A281510 A281511 * A281513 A281514 A281515

Keyword

nonn,more,hard

Author

Robert Price, Apr 13 2017

Extensions

a(28) from Robert Price, Jan 21 2019

Status

approved