OFFSET
1,1
COMMENTS
Numbers k such that the digits 35 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 351w.
EXAMPLE
3 is in this sequence because (316*10^3 - 1)/9 = 35111 is prime.
Initial terms and associated primes:
a(1) = 2, 3511;
a(2) = 3, 35111;
a(3) = 8, 3511111111;
a(4) = 9, 35111111111;
a(5) = 11, 3511111111111; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(316*10^# - 1)/9] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Jul 21 2017
STATUS
approved