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%I #21 Jul 08 2021 06:20:52
%S 3,73,773,77773,777777773,777777777773,777777777777773,
%T 777777777777777777773
%N Primes of the form 70*R_k + 3, where R_k is the repunit (A002275) of length k.
%C Primes of the form (7*10^k - 43)/9. - _Vincenzo Librandi_, Nov 16 2010
%C The next term, a(9), has 264 digits. - _Harvey P. Dale_, Jul 19 2012
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/7/77773.htm#prime">Prime numbers of the form 77...773</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%t Select[Table[FromDigits[PadLeft[{3},n,7]],{n,500}],PrimeQ] (* _Harvey P. Dale_, Jul 19 2012 *)
%Y Cf. A056689 (corresponding k), A099420.
%K nonn
%O 1,1
%A _Rick L. Shepherd_, Mar 26 2004
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