A294908 Numbers k such that (13*10^k + 107)/3 is prime.

1, 5, 7, 9, 16, 39, 87, 127, 129, 132, 226, 228, 327, 772, 1191, 1689, 4953, 5143, 25121, 26327, 52861, 60966

Offset

1,2

Comments

For k > 1, numbers such that the digit 4 followed by k-2 occurrences of the digit 3 followed by the digits 69 is prime (see Example section).

a(23) > 2*10^5.

Links

Table of n, a(n) for n=1..22.

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

Makoto Kamada, Search for 43w69

Example

5 is in this sequence because (13*10^5 + 107)/3 = 433369 is prime.
Initial terms and primes associated:
a(1) = 1, 79;
a(2) = 5, 433369;
a(3) = 7, 43333369;
a(4) = 9, 4333333369;
a(5) = 16, 43333333333333369; etc.

Mathematica

Select[Range[1, 100000], PrimeQ[(13*10^# + 107)/3] &]

Crossrefs

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

Sequence in context: A177941 A029667 A264747 * A036708 A124822 A072180

Adjacent sequences: A294905 A294906 A294907 * A294909 A294910 A294911

Keyword

nonn,more,hard

Author

Robert Price, Nov 10 2017

Status

approved