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 93 is prime (see Example section).
a(25) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 16w93.
EXAMPLE
2 is in this sequence because (5*10^2 + 79)/3 = 193 is prime.
Initial terms and associated primes:
a(1) = 1, 43;
a(2) = 2, 193;
a(3) = 3, 1693;
a(4) = 4, 16693;
a(5) = 5, 166693; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(5*10^# + 79)/3] &]
PROG
(PARI) is(k) = ispseudoprime((5*10^k + 79)/3) \\ Iain Fox, Nov 12 2017
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Nov 12 2017
EXTENSIONS
a(24) from Robert Price, Mar 09 2018
STATUS
approved