A295082 Numbers k such that (17*10^k - 521)/9 is prime.

2, 3, 5, 8, 9, 14, 20, 21, 29, 41, 113, 677, 3011, 3015, 3083, 3140, 4473, 4947, 138812

Offset

1,1

Comments

For k > 1, numbers k such that the digit 1 followed by k-2 occurrences of the digit 8 followed by the digits 31 is prime (see Example section).

a(20) > 2*10^5.

Links

Table of n, a(n) for n=1..19.

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

Makoto Kamada, Search for 18w31.

Example

2 is in this sequence because (17*10^2 - 521)/9 = 131 is prime.
Initial terms and associated primes:
a(1) = 2, 131;
a(2) = 3, 1831;
a(3) = 5, 188831;
a(4) = 8, 188888831;
a(5) = 9, 1888888831; etc.

Mathematica

Select[Range[2, 100000], PrimeQ[(17*10^# - 521)/9] &]

Prog

(PARI) isok(k) = isprime((17*10^k - 521)/9); \\ Michel Marcus, Nov 14 2017

Crossrefs

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

Sequence in context: A141399 A104737 A286495 * A168154 A278334 A120057

Adjacent sequences: A295079 A295080 A295081 * A295083 A295084 A295085

Keyword

nonn,more,hard

Author

Robert Price, Nov 13 2017

Extensions

a(19) from Robert Price, Apr 24 2018

Status

approved