OFFSET
1,3
COMMENTS
For k > 1, numbers k such that the digit 6 followed by k-2 occurrences of the digit 2 followed by the digits 99 is prime (see Example section).
a(28) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 62w99.
EXAMPLE
3 is in this sequence because (56*10^3 + 691)/9 = 6299 is prime.
Initial terms and associated primes:
a(1) = 0, 83;
a(2) = 1, 139;
a(3) = 3, 6299;
a(4) = 4, 62299;
a(5) = 6, 6222299; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(56*10^# + 691)/9] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Sep 04 2017
EXTENSIONS
a(24)-a(27) from Robert Price, May 03 2019
STATUS
approved