OFFSET
1,1
COMMENTS
For k > 1, numbers k such that the digit 3 followed by k-2 occurrences of the digit 5 followed by the digits 47 is prime (see Example section).
a(25) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 35w47.
EXAMPLE
3 is in this sequence because (32*10^3 - 77)/9 = 3547 is prime.
Initial terms and associated primes:
a(1) = 2, 347;
a(2) = 3, 3547;
a(3) = 8, 355555547;
a(4) = 12; 3555555555547;
a(5) = 15, 3555555555555547; etc.
MATHEMATICA
Select[Range[1, 100000], PrimeQ[(32*10^# - 77)/9] &]
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Jul 21 2017
EXTENSIONS
a(22)-a(24) from Robert Price, Jun 10 2018
STATUS
approved