OFFSET
1,3
COMMENTS
Numbers k such that the digits 19 followed by k occurrences of the digit 7 is prime (see Example section).
a(23) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 197w.
EXAMPLE
3 is in this sequence because (178*10^3 - 7)/9 = 19777 is prime.
Initial terms and associated primes:
a(1) = 0, 19;
a(2) = 1, 197;
a(3) = 3, 19777;
a(4) = 7; 197777777;
a(5) = 12, 19777777777777; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(178*10^# - 7)/9] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Aug 14 2017
STATUS
approved