OFFSET
1,3
COMMENTS
For k > 1, numbers k such that the digit 1 followed by k-2 occurrences of the digit 3 followed by the digits 61 is prime (see Example section).
a(31) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 13w61.
EXAMPLE
3 is in this sequence because (4*10^3 + 83) / 3 = 1361 is prime.
Initial terms and associated primes:
a(1) = 0, 29;
a(2) = 1, 41;
a(3) = 3, 1361;
a(4) = 7, 13333361;
a(5) = 9, 1333333361; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(4*10^# + 83) / 3] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Jan 09 2017
EXTENSIONS
a(27)-a(30) from Robert Price, Feb 02 2018
STATUS
approved