OFFSET
1,3
COMMENTS
For k > 1, numbers k such that k-2 occurrences of the digit 6 followed by the digits 83 is prime (see Example section).
a(27) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 6w83.
EXAMPLE
3 is in this sequence because (2*10^3 + 49) / 3 = 683 is prime.
Initial terms and associated primes:
a(1) = 0, 17;
a(2) = 1, 23;
a(3) = 2, 83;
a(4) = 3, 683;
a(5) = 5, 66683; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(2*10^# + 49) / 3] &]
PROG
(PARI) is(n)=ispseudoprime((2*10^n + 49)/3) \\ Charles R Greathouse IV, Jun 13 2017
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Jan 04 2017
STATUS
approved