OFFSET
1,3
COMMENTS
Numbers k such that the digits 19 followed by k occurrences of the digit 3 is prime (see Example section).
a(25) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 193w.
EXAMPLE
2 is in this sequence because (58*10^2 - 1) / 3 = 1933 is prime.
Initial terms and associated primes:
a(1) = 0, 19;
a(2) = 1, 193;
a(3) = 2, 1933;
a(4) = 3, 19333;
a(5) = 25, 193333333333333333333333333; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(58*10^# - 1) / 3] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Jan 16 2017
STATUS
approved