OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 7 followed by k-2 occurrences of the digit 9 followed by the digits 87 is prime (see Example section).
a(31) > 3*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 79w87.
EXAMPLE
4 is in this sequence because 8*10^4 - 13 = 79987 is prime.
Initial terms and associated primes:
a(1) = 1, 67;
a(2) = 2, 787;
a(3) = 4, 79987;
a(4) = 7, 79999987;
a(5) = 8, 799999987; etc.
MATHEMATICA
Select[Range[1, 100000], PrimeQ[8*10^# - 13] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, May 21 2017
EXTENSIONS
a(26)-a(28) from Robert Price, Aug 08 2019
a(29)-a(30) from Robert Price, Oct 26 2023
STATUS
approved