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%I #31 Dec 12 2020 15:39:42
%S 7,97,997,99999999999999997
%N Primes of the form 10^n - 3.
%C Primes of the form (9*10^n - 27)/9. - _Vincenzo Librandi_, Nov 16 2010
%C Also primes of the form 9*R_n - 2, where R_n is the repunit (A002275) of length n.
%C The next term has 140 digits.
%C a(n) = 10^A089675(n) - 3 = 10^(A056662(n) + 1) - 3. - _Farideh Firoozbakht_, Nov 27 2013
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/9/99997.htm#prime">Prime numbers of the form 99...997</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%t Do[If[PrimeQ[10^n - 3], Print[10^n - 3]], {n, 100}] (* _Farideh Firoozbakht_, Nov 27 2013 *)
%t Select[Table[FromDigits[PadLeft[{7},n,9]],{n,25}],PrimeQ] (* _Harvey P. Dale_, Dec 12 2020 *)
%Y Cf. A002275, A056662, A089675.
%K nonn
%O 1,1
%A _Rick L. Shepherd_, Mar 26 2004
%E Name shortened and old name moved to comments by _Alex Ratushnyak_, Apr 26 2012
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