OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 4 followed by k-2 occurrences of the digit 3 followed by the digits 09 is prime (see Example section).
a(26) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 43w09.
EXAMPLE
5 is in this sequence because (13*10^5 - 73) / 3 = 433309 is prime.
Initial terms and associated primes:
a(1) = 1, 19;
a(2) = 2, 409;
a(3) = 5, 433309;
a(4) = 9, 4333333309;
a(5) = 12, 4333333333309; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(13*10^# - 73) / 3] &]
PROG
(PARI) is(n)=ispseudoprime((13*10^n - 73)/3) \\ Charles R Greathouse IV, Jun 13 2017
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Dec 07 2016
STATUS
approved