A291202 Numbers k such that (43*10^k + 83)/9 is prime.

2, 3, 8, 11, 17, 41, 57, 62, 77, 101, 329, 333, 359, 365, 968, 1169, 1190, 1772, 2237, 12075, 30848, 63200, 190547

Offset

1,1

Comments

For k>1, numbers such that the digit 4 followed by k-2 occurrences of the digit 7 followed by the digits 87 is prime (see Example section).

a(24) > 2*10^5.

Links

Table of n, a(n) for n=1..23.

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

Makoto Kamada, Search for 47w87.

Example

3 is in this sequence because (43*10^3 + 83)/9 = 4787 is prime.
Initial terms and primes associated:
a(1) = 2, 487;
a(2) = 3, 4787;
a(3) = 8, 477777787;
a(4) = 11, 477777777787;
a(5) = 17, 477777777777777787; etc.

Mathematica

Select[Range[0, 100000], PrimeQ[(43*10^# + 83)/9] &]

Prog

(Magma) [n: n in [1..300] |IsPrime((43*10^n + 83) div 9)]; // Vincenzo Librandi, Aug 21 2017

Crossrefs

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

Sequence in context: A212771 A119116 A138756 * A091076 A112595 A041075

Adjacent sequences: A291199 A291200 A291201 * A291203 A291204 A291205

Keyword

nonn,more,hard

Author

Robert Price, Aug 20 2017

Extensions

a(23) from Robert Price, Nov 28 2018

Status

approved