OFFSET
1,1
COMMENTS
For k > 1, numbers k such that the digit 6 followed by k-2 occurrences of the digit 7 followed by the digits 91 is prime (see Example section).
a(23) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 67w91.
EXAMPLE
3 is in this sequence because (61*10^3 + 119)/9 = 179 is prime.
Initial terms and associated primes:
a(1) = 2, 691;
a(2) = 3, 6791;
a(3) = 5, 677791;
a(4) = 9, 6777777791;
a(5) = 11, 677777777791; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(61*10^# + 119)/9] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Sep 28 2017
EXTENSIONS
a(22) from Robert Price, May 23 2019
STATUS
approved