OFFSET
1,1
COMMENTS
For k > 1, numbers k such that the digit 1 followed by k-2 occurrences of the digit 9 followed by the digits 31 is prime (see Example section).
a(23) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 19w31.
EXAMPLE
3 is in this sequence because 2*10^3 - 69 = 1931 is prime.
Initial terms and associated primes:
a(1) = 2, 131;
a(2) = 3, 1931;
a(3) = 5, 199931;
a(4) = 8, 199999931;
a(5) = 30, 1999999999999999999999999999931; etc.
MATHEMATICA
Select[Range[1, 100000], PrimeQ[2*10^# - 69] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Jul 18 2017
EXTENSIONS
a(22) from Robert Price, Mar 22 2018
STATUS
approved