A294489 Numbers k such that (77*10^k - 59)/9 is prime.

0, 1, 4, 6, 12, 28, 39, 58, 73, 102, 141, 409, 423, 567, 831, 930, 1515, 2619, 5727, 9235, 12706, 13189, 37182, 73917

Offset

1,3

Comments

For k > 1, numbers k such that the digit 8 followed by k-2 occurrences of the digit 5 followed by the digits 49 is prime (see Example section).

a(25) > 2*10^5.

Links

Table of n, a(n) for n=1..24.

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

Makoto Kamada, Search for 85w49.

Example

4 is in this sequence because (77*10^4 - 59)/9 = 85549 is prime.
Initial terms and associated primes:
a(1) = 0, 2;
a(2) = 1, 79;
a(3) = 4, 85549;
a(4) = 6, 8555549;
a(5) = 12, 8555555555549; etc.

Mathematica

Select[Range[0, 100000], PrimeQ[(77*10^# - 59)/9] &]

Prog

(PARI) isok(n) = isprime((77*10^n - 59)/9); \\ Michel Marcus, Nov 01 2017

Crossrefs

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

Sequence in context: A197467 A256350 A215177 * A344995 A115076 A126259

Adjacent sequences: A294486 A294487 A294488 * A294490 A294491 A294492

Keyword

nonn,more,hard

Author

Robert Price, Oct 31 2017

Status

approved