OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 8 followed by k-2 occurrences of the digit 3 followed by the digits 57 is prime (see Example section).
a(26) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 83w57.
EXAMPLE
2 is in this sequence because (25*10^2 + 71) / 3 = 857 is prime.
Initial terms and associated primes:
a(1) = 1, 107;
a(2) = 2, 857;
a(3) = 4, 83357;
a(4) = 7, 83333357;
a(5) = 10, 83333333357; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(25*10^# + 71) / 3] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Jan 16 2017
STATUS
approved