OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 5 followed by k-2 occurrences of the digit 0 followed by the digits 81 is prime (see Example section).
a(33) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 50w81.
EXAMPLE
3 is in this sequence because 5*10^3 + 81 = 5081 is prime.
Initial terms and associated primes:
a(1) = 1, 131;
a(2) = 3, 5081;
a(3) = 6, 5000081;
a(4) = 16, 50000000000000081;
a(5) = 30, 5000000000000000000000000000081; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[5*10^# + 81] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Aug 27 2017
EXTENSIONS
a(29)-a(32) from Robert Price, Mar 04 2019
Terms reordered into ascending order by Robert Price, Apr 03 2022
STATUS
approved