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 19 is prime (see Example section).
a(31) > 3*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 43w19.
EXAMPLE
2 is in this sequence because (13*10^2 - 43)/3 = 419 is prime.
Initial terms and associated primes:
a(1) = 1, 29;
a(2) = 2, 419;
a(3) = 4, 43319;
a(4) = 5; 433319;
a(5) = 8, 433333319; etc.
MATHEMATICA
Select[Range[1, 100000], PrimeQ[(13*10^# - 43)/3] &]
PROG
(PARI) isok(n) = ispseudoprime((13*10^n - 43)/3) \\ Altug Alkan, Aug 15 2017
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Aug 15 2017
EXTENSIONS
a(25) from Robert Price, Nov 28 2018
a(26)-a(30) from Robert Price, Oct 26 2023
STATUS
approved