OFFSET
1,3
COMMENTS
For k > 1, numbers k such that the digits 43 followed by k occurrences of the digit 1 is prime (see Example section).
a(21) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 431w.
EXAMPLE
1 is in this sequence because (388*10^1 - 1)/9 = 431 is prime.
Initial terms and associated primes:
a(1) = 0, 43;
a(2) = 1, 431;
a(3) = 7, 431111111;
a(4) = 12, 43111111111111;
a(5) = 16, 431111111111111111; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(388*10^# - 1)/9] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Nov 24 2017
STATUS
approved