A295397 Numbers k such that (35*10^k - 737)/9 is prime.

2, 7, 13, 20, 26, 73, 103, 268, 383, 527, 1748, 2848, 4424, 7933, 8311, 9700, 14872, 18218, 31294, 42032, 55547, 111232

Offset

1,1

Comments

For k > 1, numbers such that the digit 3 followed by k-2 occurrences of the digit 8 followed by the digits 07 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 38w07

Example

2 is in this sequence because (35*10^2 - 737)/9 = 307 is prime.
Initial terms and primes associated:
a(1) = 2, 307;
a(2) = 7, 38888807;
a(3) = 13, 38888888888807;
a(4) = 20, 388888888888888888807;
a(5) = 26, 388888888888888888888888807; etc.

Mathematica

Select[Range[2, 100000], PrimeQ[(35*10^# - 737)/9] &]

Prog

(PARI) isok(k) = isprime((35*10^k - 737)/9); \\ Michel Marcus, Nov 22 2017

Crossrefs

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

Sequence in context: A079382 A111354 A344959 * A329817 A155212 A346870

Adjacent sequences: A295394 A295395 A295396 * A295398 A295399 A295400

Keyword

nonn,more,hard

Author

Robert Price, Nov 21 2017

Extensions

a(22) from Robert Price, Jul 16 2018

Status

approved