OFFSET
1,1
COMMENTS
For k > 1, numbers k such that k-2 occurrences of the digit 6 followed by the digits 19 is prime (see Example section).
a(26) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 6w19.
EXAMPLE
3 is in this sequence because (2*10^3 - 143)/3 = 619 is prime.
Initial terms and associated primes:
a(1) = 2, 19;
a(2) = 3, 619;
a(3) = 4, 6619;
a(4) = 9, 666666619;
a(5) = 12, 666666666619; etc.
MATHEMATICA
Select[Range[2, 100000], PrimeQ[(2*10^# - 143)/3] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Feb 04 2017
EXTENSIONS
a(24)-a(25) from Robert Price, Mar 14 2018
STATUS
approved