OFFSET
1,1
COMMENTS
For k > 0, numbers k such that the digits 11 followed by k-1 occurrences of the digit 8 followed by the digit 7 is prime (see Example section).
a(25) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 118w7.
EXAMPLE
3 is in this sequence because (107*10^3 - 17)/9 = 11887 is prime.
Initial terms and associated primes:
a(1) = 2, 1187;
a(2) = 3, 11887;
a(3) = 8, 1188888887;
a(4) = 12, 11888888888887;
a(5) = 24, 11888888888888888888888887; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(107*10^# - 17)/9] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Feb 10 2017
STATUS
approved