OFFSET
1,2
COMMENTS
For k > 0, numbers k such that the digits 96 followed by k-1 occurrences of the digit 9 followed by the digit 7 is prime (see Example section).
a(31) > 3*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 969w7.
EXAMPLE
3 is in this sequence because 97*10^3 - 3 = 96997 is prime.
Initial terms and associated primes:
a(1) = 1, 967;
a(2) = 2, 9697;
a(3) = 3, 96997;
a(4) = 8, 9699999997;
a(5) = 9, 96999999997; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[97*10^# - 3] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Jan 14 2017
EXTENSIONS
a(29)-a(30) from Robert Price, May 26 2020
STATUS
approved