OFFSET
1,2
COMMENTS
Numbers k such that the digits 54 followed by k occurrences of the digit 7 is prime (see Example section).
a(28) > 10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 547w.
EXAMPLE
5 is in this sequence because (493*10^5-7)/9 = 5477777 is prime.
Initial terms and associated primes:
a(1) = 1, 547;
a(2) = 2, 5477;
a(3) = 5, 5477777;
a(4) = 7, 547777777;
a(5) = 8, 5477777777, etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(493*10^#-7)/9] &]
PROG
(Magma) [n: n in [0..500] | IsPrime((493*10^n-7) div 9)]; // Vincenzo Librandi, Jul 27 2016
(PARI) is(n)=ispseudoprime((493*10^n-7)/9) \\ Charles R Greathouse IV, Jun 13 2017
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Robert Price, Jul 26 2016
STATUS
approved