A093946 - OEIS

%I #14 Sep 08 2022 08:45:13

%S 5,59,599,59999,599999,59999999,59999999999,59999999999999,

%T 59999999999999999999999,599999999999999999999999,

%U 59999999999999999999999999999,59999999999999999999999999999999999

%N Primes of the form 6*10^n - 1.

%C Equivalently, primes of the form 5*10^n + 9*R_n, where R_n is the repunit (A002275) of length n.

%H Makoto Kamada, <a href="https://stdkmd.net/nrr/5/59999.htm#prime">Prime numbers of the form 599...99</a>.

%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.

%t Select[Table[FromDigits[PadRight[{5},n,9]],{n,40}],PrimeQ] (* _Harvey P. Dale_, Jun 06 2016 *)

%o (Magma) [a: n in [0..200] | IsPrime(a) where a is (6*10^n-1)]; // _Vincenzo Librandi_, May 08 2019

%Y Cf. A056716 (corresponding n).

%K nonn

%O 1,1

%A _Rick L. Shepherd_, Apr 17 2004