A281142 Numbers k such that (25*10^k - 13)/3 is prime.

1, 2, 3, 6, 8, 18, 21, 33, 56, 301, 381, 426, 769, 1123, 1638, 2037, 2660, 3284, 4723, 5686, 17227, 22084, 25009, 36176, 125076, 141019

Offset

1,2

Comments

For k > 1, numbers k such that the digit 8 followed by k-2 occurrences of the digit 3 followed by the digits 29 is prime (see Example section).

a(27) > 2*10^5.

Links

Table of n, a(n) for n=1..26.

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

Makoto Kamada, Search for 83w29.

Example

3 is in this sequence because (25*10^3 - 13) / 3 = 8329 is prime.
Initial terms and associated primes:
a(1) = 1, 79;
a(2) = 2, 829;
a(3) = 3, 8329;
a(4) = 6, 8333329;
a(5) = 8, 833333329; etc.

Mathematica

Select[Range[0, 100000], PrimeQ[(25*10^# - 13) / 3] &]

Crossrefs

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

Sequence in context: A368687 A330442 A103065 * A110448 A005508 A022542

Adjacent sequences: A281139 A281140 A281141 * A281143 A281144 A281145

Keyword

nonn,more,hard,changed

Author

Robert Price, Jan 15 2017

Extensions

a(25)-a(26) from Robert Price, Sep 23 2019

Status

approved