OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digits 99 followed by k-1 occurrences of the digit 6 followed by the digit 7 is prime (see Example section).
a(29) > 3*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 996w7.
EXAMPLE
2 is in this sequence because (299*10^2 + 1)/3 = 9967 is prime.
Initial terms and associated primes:
a(1) = 1, 997;
a(2) = 2, 9967;
a(3) = 3, 99667;
a(4) = 5, 9966667;
a(5) = 6, 99666667; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(299*10^# + 1)/3] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Nov 10 2017
EXTENSIONS
a(26)-a(27) from Robert Price, Apr 27 2020
a(28) from Robert Price, Jun 21 2023
STATUS
approved