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 37 is prime (see Example section).
a(25) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 54w37.
EXAMPLE
4 is in this sequence because (49*10^4 - 67)/9 = 54437 is prime.
Initial terms and associated primes:
a(1) = 1, 47;
a(2) = 3, 5437;
a(3) = 4, 54437;
a(4) = 7, 54444437;
a(5) = 10, 54444444437; etc.
MATHEMATICA
Select[Range[1, 100000], PrimeQ[(49*10^# - 67)/9] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Aug 27 2017
EXTENSIONS
a(24) from Robert Price, Mar 15 2019
STATUS
approved