OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 3 followed by k-2 occurrences of the digit 2 followed by the digits 51 is prime (see Example section).
a(25) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 32w51.
EXAMPLE
3 is in this sequence because (29*10^3 + 259)/9 = 3251 is prime.
Initial terms and associated primes:
a(1) = 1, 61;
a(2) = 3, 3251;
a(3) = 4, 32251;
a(4) = 6, 3222251;
a(5) = 9, 3222222251; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(29*10^# + 259)/9] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Apr 01 2017
STATUS
approved