OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 5 followed by k-2 occurrences of the digit 1 followed by the digits 69 is prime (see Example section).
a(25) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 51w69.
EXAMPLE
4 is in this sequence because (46*10^4 + 521)/9 = 51169 is prime.
Initial terms and associated primes:
a(1) = 1, 109;
a(2) = 2, 569;
a(3) = 4, 51169;
a(4) = 5, 511169;
a(5) = 8, 511111169; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(46*10^# + 521)/9] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Aug 27 2017
EXTENSIONS
a(24) from Robert Price, Feb 23 2019
STATUS
approved