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 47 is prime (see Example section).
a(22) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 18w47.
EXAMPLE
3 is in this sequence because (17*10^3 - 377)/9 = 1847 is prime.
Initial terms and associated primes:
a(1) = 3, 1847;
a(2) = 19, 18888888888888888847;
a(3) = 27, 1888888888888888888888888847; etc.
MATHEMATICA
Select[Range[2, 100000], PrimeQ[(17*10^# - 377)/9] &]
PROG
(PARI) isok(k) = isprime((17*10^k - 377)/9); \\ Michel Marcus, Nov 20 2017
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Nov 19 2017
EXTENSIONS
a(21) from Robert Price, Mar 24 2018
STATUS
approved