OFFSET
1,1
COMMENTS
For k > 1, numbers k such that the digit 5 followed by k-2 occurrences of the digit 2 followed by the digits 09 is prime (see Example section).
a(26) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 52w09.
EXAMPLE
3 is in this sequence because (47*10^3 - 119)/9 = 5209 is prime.
Initial terms and associated primes:
a(1) = 2, 509;
a(2) = 3, 5209;
a(3) = 9, 5222222209;
a(4) = 15, 5222222222222209;
a(5) = 39, 5222222222222222222222222222222222222209; etc.
MATHEMATICA
Select[Range[1, 100000], PrimeQ[(47*10^# - 119)/9] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Sep 04 2017
EXTENSIONS
a(22)-a(25) from Robert Price, May 30 2019
STATUS
approved