OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 8 followed by k-2 occurrences of the digit 9 followed by the digits 71 is prime (see Example section).
a(29) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 89w71.
EXAMPLE
3 is in this sequence because 9*10^3 - 29 = 8971 is prime.
Initial terms and associated primes:
a(1) = 1, 61;
a(2) = 3, 8971;
a(3) = 5, 899971;
a(4) = 6, 8999971;
a(5) = 15, 8999999999999971; etc.
MATHEMATICA
Select[Range[1, 100000], PrimeQ[9*10^# - 29] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, May 29 2017
EXTENSIONS
a(28) from Robert Price, Sep 25 2019
STATUS
approved