OFFSET
1,3
COMMENTS
For k > 1, numbers k such that the digit 5 followed by k-2 occurrences of the digit 7 followed by the digits 91 is prime (see Example section).
a(23) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 57w91.
EXAMPLE
3 is in this sequence because (52*10^3 + 119)/9 = 5791 is prime.
Initial terms and associated primes:
a(1) = 0, 19;
a(2) = 1, 71;
a(3) = 3, 5791;
a(4) = 4, 57791;
a(5) = 9, 5777777791; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(52*10^# + 119)/9] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Oct 25 2017
STATUS
approved