OFFSET
1,1
COMMENTS
For k > 1, numbers k such that the digits 38 followed by k occurrences of the digit 7 is prime (see Example section).
a(26) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 387w.
EXAMPLE
2 is in this sequence because (349*10^2 - 7)/9 = 3877 is prime.
Initial terms and associated primes:
a(1) = 2, 3877;
a(2) = 12, 38777777777777;
a(3) = 21, 38777777777777777777777;
a(4) = 30, 38777777777777777777777777777777;
a(5) = 63, 38777777777777777777777777777777777777777777777777777777777777777; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(349*10^# - 7)/9] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Mar 29 2017
STATUS
approved