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 49 is prime (see Example section).
a(31) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 40w49.
EXAMPLE
3 is in this sequence because 4*10^3 + 49 = 4049 is prime.
Initial terms and associated primes:
a(1) = 0, 53;
a(2) = 1, 89;
a(3) = 2, 449;
a(4) = 3; 4049;
a(5) = 8, 400000049; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[4*10^# + 49] &]
PROG
(PARI) isok(k) = ispseudoprime(4*10^k + 49) \\ Altug Alkan, Aug 13 2017
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Aug 13 2017
EXTENSIONS
a(29)-a(30) from Robert Price, Jan 21 2019
STATUS
approved