OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 2 followed by k-2 occurrences of the digit 2 followed by the digits 57 is prime (see Example section).
a(24) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 21w57.
EXAMPLE
2 is in this sequence because (19*10^2 + 413)/9 = 257 is prime.
Initial terms and associated primes:
a(1) = 1, 67;
a(2) = 2, 257;
a(3) = 4, 21157;
a(4) = 7, 21111157;
a(5) = 11, 211111111157; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(19*10^# + 413)/9] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Oct 14 2017
STATUS
approved