OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 6 followed by k-2 occurrences of the digit 4 followed by the digits 91 is prime (see Example section).
a(25) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 64w91.
EXAMPLE
2 is in this sequence because (58*10^2 + 419)/9 = 691 is prime.
Initial terms and associated primes:
a(1) = 0, 53;
a(2) = 2, 691;
a(3) = 3, 6491;
a(4) = 5, 644491;
a(5) = 6, 6444491; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(58*10^# + 419)/9] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Nov 05 2017
EXTENSIONS
a(24) from Robert Price, May 18 2019
STATUS
approved