OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 5 followed by k-2 occurrences of the digit 4 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 54w79.
EXAMPLE
3 is in this sequence because (49*10^3 + 311)/9 = 5479 is prime.
Initial terms and associated primes:
a(1) = 1, 89;
a(2) = 3, 5479;
a(3) = 6, 5444479;
a(4) = 7, 54444479;
a(5) = 9, 5444444479; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(49*10^# + 311)/9] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Feb 06 2017
EXTENSIONS
a(25) from Robert Price, Jun 13 2019
STATUS
approved