OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 8 followed by k-2 occurrences of the digit 1 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 81w57.
EXAMPLE
4 is in this sequence because (73*10^4 + 413)/9 = 81157 is prime.
Initial terms and associated primes:
a(1) = 1, 127;
a(2) = 2, 857;
a(3) = 4, 81157;
a(4) = 5, 811157;
a(5) = 7, 81111157; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(73*10^# + 413)/9] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Oct 06 2017
STATUS
approved