A280060 Numbers k such that (2*10^k + 49)/3 is prime.

0, 1, 2, 3, 5, 6, 7, 8, 13, 21, 22, 25, 26, 38, 200, 395, 442, 561, 908, 1295, 5541, 7795, 8600, 19157, 22536, 45636

Offset

1,3

Comments

For k > 1, numbers k such that k-2 occurrences of the digit 6 followed by the digits 83 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 6w83.

Example

3 is in this sequence because (2*10^3 + 49) / 3 = 683 is prime.
Initial terms and associated primes:
a(1) = 0, 17;
a(2) = 1, 23;
a(3) = 2, 83;
a(4) = 3, 683;
a(5) = 5, 66683; etc.

Mathematica

Select[Range[0, 100000], PrimeQ[(2*10^# + 49) / 3] &]

Prog

(PARI) is(n)=ispseudoprime((2*10^n + 49)/3) \\ Charles R Greathouse IV, Jun 13 2017

Crossrefs

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

Sequence in context: A068526 A287339 A039086 * A197433 A153781 A255398

Adjacent sequences: A280057 A280058 A280059 * A280061 A280062 A280063

Keyword

nonn,more,hard

Author

Robert Price, Jan 04 2017

Status

approved