OFFSET
1,2
COMMENTS
For k > 0, numbers k such that the digits 16 followed by k occurrences of the digit 7 (see Example section).
a(25) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 167w.
EXAMPLE
4 is in this sequence because (151*10^4 - 7)/9 = 167777 is prime.
Initial terms and associated primes:
a(1) = 1, 167;
a(2) = 4, 167777;
a(3) = 6, 16777777;
a(4) = 12, 16777777777777;
a(5) = 13, 167777777777777; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(151*10^# - 7)/9] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Jul 07 2017
EXTENSIONS
a(24) from Robert Price, Jan 08 2020
STATUS
approved