OFFSET
1,1
COMMENTS
For k > 1, numbers k such that the digit 1 followed by k-2 occurrences of the digit 9 followed by the digits 57 is prime (see Example section).
a(27) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 19w57.
EXAMPLE
6 is in this sequence because 2*10^6 - 43 = 1999957 is prime.
Initial terms and associated primes:
a(1) = 2, 157;
a(2) = 6, 1999957;
a(3) = 8, 199999957;
a(4) = 13, 19999999999957;
a(5) = 24, 1999999999999999999999957; etc.
MATHEMATICA
Select[Range[2, 100000], PrimeQ[2*10^# - 43] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Jul 11 2017
EXTENSIONS
a(22)-a(26) from Robert Price, Apr 11 2018
STATUS
approved