OFFSET
1,2
COMMENTS
Numbers k such that the digits 96 followed by k occurrences of the digit 7 is prime (see Example section).
a(22) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 967w.
EXAMPLE
2 is in this sequence because (871*10^2 - 7)/9 = 9677 is prime.
Initial terms and associated primes:
a(1) = 1, 967;
a(2) = 2, 9677;
a(3) = 7, 967777777;
a(4) = 26, 9677777777777777777777777777;
a(5) = 28, 967777777777777777777777777777; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(871*10^# - 7)/9] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Nov 06 2017
STATUS
approved