OFFSET
1,1
COMMENTS
For k > 1, numbers k such that the digits 53 followed by k-1 occurrences of the digit 5 followed by the digit 1 is prime (see Example section).
a(22) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 535w1.
EXAMPLE
3 is in this sequence because (482*10^3 - 41)/9 = 53551 is prime.
Initial terms and associated primes:
a(1) = 2, 5351;
a(2) = 3, 53551;
a(3) = 5, 5355551;
a(4) = 14, 5355555555555551;
a(5) = 17, 5355555555555555551; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(482*10^# - 41)/9] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Sep 05 2017
STATUS
approved