OFFSET
1,1
COMMENTS
For k > 1, numbers k such that k-2 occurrences of the digit 7 followed by the digits 43 is prime (see Example section).
a(27) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 7w43.
EXAMPLE
3 is in this sequence because (7*10^3 - 313) / 9 = 743 is prime.
Initial terms and associated primes:
a(1) = 2, 43;
a(2) = 3, 743;
a(3) = 5, 77743;
a(4) = 6, 777743;
a(5) = 9, 777777743; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(7*10^# - 313) / 9] &]
PROG
(PARI) isok(n) = isprime((7*10^n - 313) / 9); \\ Michel Marcus, Nov 27 2016
(Magma) [n: n in [0..500] | IsPrime((7*10^n-313) div 9)]; // Vincenzo Librandi, Nov 27 2016
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Nov 26 2016
STATUS
approved