OFFSET
1,1
COMMENTS
For k > 1, numbers k such that the digit 4 followed by k-2 occurrences of the digit 7 followed by the digits 87 is prime (see Example section).
a(24) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 47w87.
EXAMPLE
3 is in this sequence because (43*10^3 + 83)/9 = 4787 is prime.
Initial terms and associated primes:
a(1) = 2, 487;
a(2) = 3, 4787;
a(3) = 8, 477777787;
a(4) = 11, 477777777787;
a(5) = 17, 477777777777777787; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(43*10^# + 83)/9] &]
PROG
(Magma) [n: n in [1..300] |IsPrime((43*10^n + 83) div 9)]; // Vincenzo Librandi, Aug 21 2017
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Aug 20 2017
EXTENSIONS
a(23) from Robert Price, Nov 28 2018
STATUS
approved