OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 3 followed by k-2 occurrences of the digit 5 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 35w91.
EXAMPLE
4 is in this sequence because (32*10^4 + 319)/9 = 35591 is prime.
Initial terms and associated primes:
a(1) = 1, 71;
a(2) = 4, 35591;
a(3) = 5, 355591;
a(4) = 7, 35555591;
a(5) = 8, 355555591; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(32*10^# + 319)/9] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Oct 17 2017
STATUS
approved