OFFSET
1,2
COMMENTS
For k > 1, numbers k such that k-2 occurrences of the digit 5 followed by the digits 79 is prime (see Example section).
a(23) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 5w79.
EXAMPLE
2 is in this sequence because (5*10^2 + 211)/9 = 79 is prime.
Initial terms and associated primes:
a(1) = 1, 29;
a(2) = 2, 79;
a(3) = 5, 55579;
a(4) = 8, 55555579;
a(5) = 32, 55555555555555555555555555555579; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(5*10^# + 211)/9] &]
PROG
(PARI) isok(k) = isprime((5*10^k + 211)/9); \\ Michel Marcus, Dec 01 2017
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Nov 30 2017
EXTENSIONS
a(22) from Robert Price, Feb 14 2018
STATUS
approved