OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 2 followed by k-2 occurrences of the digit 3 followed by the digits 69 is prime (see Example section).
a(29) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 23w69.
EXAMPLE
4 is in this sequence because (7*10^4 + 107)/3 = 23369 is prime.
Initial terms and associated primes:
a(1) = 1, 59;
a(2) = 2, 269;
a(3) = 4, 23369;
a(4) = 8, 233333369;
a(5) = 12, 2333333333369; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(7*10^# + 107)/3] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Feb 20 2017
EXTENSIONS
a(25)-a(28) from Robert Price, Feb 21 2018
STATUS
approved