OFFSET
1,1
COMMENTS
For k > 1, numbers k such that the digit 3 followed by k-2 occurrences of the digit 7 followed by the digits 33 is prime (see Example section).
a(21) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 37w33.
EXAMPLE
3 is in this sequence because (34*10^3 - 403)/9 = 3733 is prime.
Initial terms and associated primes:
a(1) = 3, 3733;
a(2) = 24, 3777777777777777777777733;
a(3) = 39, 3777777777777777777777777777777777777733;
a(4) = 58, 37777777777777777777777777777777777777777777777777777777733; etc.
MATHEMATICA
Select[Range[2, 100000], PrimeQ[(34*10^# - 403)/9] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Oct 12 2017
STATUS
approved