OFFSET
1,2
COMMENTS
For k > 0, numbers k such that the digits 46 followed by k-1 occurrences of the digit 4 followed by the digit 7 is prime. See Example section.
a(26) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 464w7.
EXAMPLE
3 is in this sequence because (418*10^3 + 23)/9 = 46447 is prime.
Initial terms and associated primes:
a(1) = 1, 467;
a(2) = 3, 46447;
a(3) = 4, 464447;
a(4) = 7, 464444447;
a(5) = 16, 464444444444444447; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(418*10^# + 23)/9] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Aug 30 2017
EXTENSIONS
a(24)-a(25) from Robert Price, Feb 05 2020
STATUS
approved