OFFSET
1,1
COMMENTS
For k > 1, numbers k such that the digit 1 followed by k-2 occurrences of the digit 5 followed by the digits 79 is prime (see Example section).
a(26) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 15w79.
EXAMPLE
2 is in this sequence because (14*10^2 + 211)/9 = 179 s prime.
Initial terms and associated primes:
a(1) = 2, 179;
a(2) = 3, 1579;
a(3) = 5, 155579;
a(4) = 6, 1555579;
a(5) = 8, 155555579; etc.
MATHEMATICA
Select[Range[0 100000], PrimeQ[(14*10^# + 211)/9] &]
PROG
(PARI) isok(k) = isprime((14*10^k + 211)/9); \\ Michel Marcus, Nov 12 2017
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Nov 11 2017
EXTENSIONS
a(25) from Robert Price, Apr 21 2018
STATUS
approved