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%I #11 Sep 08 2022 08:45:13
%S 7,79,79999,799999,799999999,79999999999,79999999999999999999999999,
%T 79999999999999999999999999999999999999999999999999,
%U 79999999999999999999999999999999999999999999999999999999999999999999999999999
%N Primes of the form 8*10^n - 1.
%C Equivalently, primes of the form 7*10^n + 9*R_n, where R_n is the repunit (A002275) of length n.
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/7/79999.htm#prime">Prime numbers of the form 799...99</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%t Select[Table[FromDigits[PadRight[{7}, n, 9]], {n, 50}], PrimeQ] (* _Vincenzo Librandi_, May 08 2019 *)
%o (Magma) [a: n in [0..200] | IsPrime(a) where a is (8*10^n-1)]; // _Vincenzo Librandi_, May 08 2019
%Y Cf. A056721 (corresponding n).
%K nonn
%O 1,1
%A _Rick L. Shepherd_, Apr 17 2004
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