OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 9 followed by k-2 occurrences of the digit 5 followed by the digits 21 is prime (see Example section).
a(25) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 95w21.
EXAMPLE
3 is in this sequence because (86*10^3 - 311)/9 = 9521 is prime.
Initial terms and associated primes:
a(1) = 1, 61;
a(2) = 3, 9521;
a(3) = 6, 9555521;
a(4) = 13, 95555555555521;
a(5) = 31, 95555555555555555555555555555521; etc.
MATHEMATICA
Select[Range[1, 100000], PrimeQ[(86*10^# - 311)/9] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Oct 19 2017
STATUS
approved