OFFSET
1,1
COMMENTS
Numbers k > 1 such that the digit 1 followed by k-2 occurrences of the digit 4 followed by the digits 79 is prime (see Example section).
a(20) > 10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 14w79.
EXAMPLE
2 is in this sequence because (13*10^2 + 311)/9 = 179 is prime.
Initial terms and associated primes:
a(1) = 2, 179;
a(2) = 4, 14479;
a(3) = 5, 144479;
a(4) = 14, 144444444444479;
a(5) = 28, 14444444444444444444444444479; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(13*10^# + 311)/9] &]
PROG
(PARI) isok(k) = isprime((13*10^k + 311)/9); \\ Michel Marcus, Nov 13 2017
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Nov 12 2017
STATUS
approved