OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 1 followed by k-2 occurrences of the digit 6 followed by the digits 57 is prime (see Example section).
a(28) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 16w57.
EXAMPLE
3 is in this sequence because (5*10^3 - 29)/3 = 1657 is prime.
Initial terms and associated primes:
a(1) = 1, 7;
a(2) = 2, 157;
a(3) = 3, 1657;
a(4) = 4, 16657;
a(5) = 5, 166657; etc.
MATHEMATICA
Select[Range[1, 100000], PrimeQ[(5*10^# - 29)/3] &]
PROG
(PARI) isok(k) = isprime((5*10^k - 29)/3); \\ Michel Marcus, Mar 06 2018
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Feb 16 2017
EXTENSIONS
a(26)-a(27) from Robert Price, Mar 05 2018
STATUS
approved