OFFSET
1,1
COMMENTS
For k > 1, numbers k such that the digit 3 followed by k-2 occurrences of the digit 9 followed by the digits 01 is prime (see Example section).
a(29) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 39w01.
EXAMPLE
4 is in this sequence because 4*10^4 - 99 = 39901 is prime.
Initial terms and associated primes:
a(1) = 4, 39901;
a(2) = 6, 3999901;
a(3) = 7, 39999901;
a(4) = 9, 3999999901;
a(5) = 43, 39999999999999999999999999999999999999999901; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[4*10^# - 99] &]
PROG
(Magma) [n: n in [2..500] | IsPrime(4*10^n - 99)]; // Vincenzo Librandi, Dec 03 2016
(PARI) is(n)=ispseudoprime(4*10^n - 99) \\ Charles R Greathouse IV, Jun 13 2017
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Dec 02 2016
EXTENSIONS
Link and Mathematica program corrected by Robert Price, Jun 11 2018
STATUS
approved