OFFSET
1,2
COMMENTS
For k > 2, numbers k such that k-2 occurrences of the digit 2 followed by the digits 73 is prime (see Example section).
a(29) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 2w73.
EXAMPLE
2 is in this sequence because (2*10^2 + 457) / 9 = 73 is prime.
Initial terms and associated primes:
a(1) = 1, 53;
a(2) = 2, 73;
a(3) = 4, 2273;
a(4) = 5, 22273;
a(5) = 7, 2222273; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(2*10^# + 457) / 9] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Jan 18 2017
EXTENSIONS
a(28) from Robert Price, Jan 08 2018
STATUS
approved