OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 4 followed by k-2 occurrences of the digit 7 followed by the digits 89 is prime (see Example section).
a(25) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 47w89.
EXAMPLE
3 is in this sequence because (43*10^3 + 101)/9 = 4789 is prime.
Initial terms and associated primes:
a(1) = 1, 59;
a(2) = 3, 4789;
a(3) = 6, 4777789;
a(4) = 9, 4777777789;
a(5) = 12, 4777777777789; etc.
MATHEMATICA
Select[Range[1, 100000], PrimeQ[(43*10^# + 101)/9] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Nov 02 2017
EXTENSIONS
a(22)-a(24) from Robert Price, Nov 30 2018
STATUS
approved