OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 8 followed by k-2 occurrences of the digit 3 followed by the digits 87 is prime (see Example section).
a(29) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 83w87.
EXAMPLE
3 is in this sequence because (25*10^3 + 161) / 3 = 8387 is prime.
Initial terms and associated primes:
a(1) = 1, 137;
a(2) = 2, 887;
a(3) = 3, 8387;
a(4) = 6, 8333387;
a(5) = 12, 8333333333387; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(25*10^# + 161) / 3] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Jan 14 2017
EXTENSIONS
a(27)-a(28) from Robert Price, Sep 30 2019
STATUS
approved