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%I #40 Mar 04 2024 18:54:28
%S 3,53,55555553,55555555555555555555555553,
%T 555555555555555555555555555555555555555555555555555555555555555553,
%U 55555555555555555555555555555555555555555555555555555555555555555555555553
%N Primes of the form 50*R_k + 3, where R_k is the repunit (A002275) of length k.
%C Primes of the form (5*10^k - 23)/9. - _Vincenzo Librandi_, Nov 16 2010
%C The next term (a(7)) has 233 digits. - _Harvey P. Dale_, Mar 04 2024
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/5/55553.htm#prime">Prime numbers of the form 55...553</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%t Select[(5(10^Range[74])-23)/9,PrimeQ] (* _Paul F. Marrero Romero_, Oct 20 2023 *)
%t Select[Table[FromDigits[PadLeft[{3},n,5]],{n,75}],PrimeQ] (* _Harvey P. Dale_, Mar 04 2024 *)
%Y Cf. A002275, A056685 (corresponding k).
%K nonn
%O 1,1
%A _Rick L. Shepherd_, Mar 26 2004
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