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
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
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Jun 09 2017
STATUS
approved