OFFSET
1,1
COMMENTS
For k > 1, numbers k such that the digit 6 followed by k-2 occurrences of the digit 9 followed by the digits 31 is prime (see Example section).
a(37) > 3*10^5. - Robert Price, Jul 10 2023
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 69w31.
EXAMPLE
4 is in this sequence because 7*10^4 - 69 = 69931 is prime.
Initial terms and associated primes:
a(1) = 2, 631;
a(2) = 4, 69931;
a(3) = 5, 699931;
a(4) = 12, 6999999999931;
a(5) = 29, 699999999999999999999999999931; etc.
MATHEMATICA
Select[Range[1, 100000], PrimeQ[7*10^# - 69] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Jan 25 2017
EXTENSIONS
a(32)-a(36) from Robert Price, Jul 03 2019
STATUS
approved