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 79 is prime (see Example section).
a(27) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 13w79.
EXAMPLE
5 is in this sequence because (4*10^5 + 137)/3 = 179 is prime.
Initial terms and associated primes:
a(1) = 0, 47;
a(2) = 1, 59;
a(3) = 2, 179;
a(4) = 5, 133379;
a(5) = 8, 133333379; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(4*10^# + 137)/3] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Jul 11 2017
EXTENSIONS
a(25)-a(26) from Robert Price, Mar 21 2018
STATUS
approved