A288483 Numbers k such that (782*10^k - 71)/9 is prime.

0, 2, 9, 14, 15, 30, 48, 57, 75, 98, 125, 483, 792, 930, 1133, 1764, 3123, 3212, 3429, 3989, 5595, 17262, 37287, 47033, 49148, 87809

Offset

1,2

Comments

For k > 0, numbers k such that the digits 86 followed by k-1 occurrences of the digit 8 followed by the digit 1 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 868w1.

Example

2 is in this sequence because (782*10^2 - 71)/9 = 8681 is prime.
Initial terms and associated primes:
a(1) = 0, 79;
a(2) = 2, 8681;
a(3) = 9, 86888888881;
a(4) = 14, 8688888888888881;
a(5) = 15, 86888888888888881; etc.

Mathematica

Select[Range[0, 100000], PrimeQ[(782*10^# - 71)/9] &]

Crossrefs

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

Sequence in context: A043401 A226953 A274133 * A353308 A304807 A045920

Adjacent sequences: A288480 A288481 A288482 * A288484 A288485 A288486

Keyword

nonn,more,hard

Author

Robert Price, Jun 09 2017

Status

approved