OFFSET
1,3
COMMENTS
For k > 1, numbers k such that the digit 4 followed by k-2 occurrences of the digit 0 followed by the digits 39 is prime (see Example section).
a(24) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 40w39.
EXAMPLE
2 is in this sequence because 4*10^2 + 39 = 439 is prime.
Initial terms and associated primes:
a(1) = 0, 43;
a(2) = 1, 79;
a(3) = 2, 439;
a(4) = 4, 40039;
a(5) = 6, 4000039; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[4*10^# + 39] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Nov 10 2017
STATUS
approved