OFFSET
1,1
COMMENTS
For k > 1, numbers k such that the digit 5 followed by k-2 occurrences of the digit 9 followed by the digits 09 is prime (see Example section).
a(29) > 3*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 59w09.
EXAMPLE
2 is in this sequence because 6*10^2 - 91 = 509 is prime.
Initial terms and associated primes:
a(1) = 2, 509;
a(2) = 6, 5999909;
a(3) = 25, 59999999999999999999999909;
a(4) = 30, 5999999999999999999999999999909;
a(5) = 45, 5999999999999999999999999999999999999999999909; etc.
MATHEMATICA
Select[Range[2, 100000], PrimeQ[6*10^# - 91] &]
PROG
(PARI) isok(k) = isprime(6*10^k - 91); \\ Michel Marcus, Nov 12 2017
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Nov 11 2017
EXTENSIONS
Terms reordered into ascending order by Robert Price, Apr 03 2022
a(28) from Robert Price, Jul 12 2023
STATUS
approved