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%I #21 Dec 19 2022 14:19:32
%S 3,43,443,444443,444444443,444444444443,
%T 444444444444444444444444444443,44444444444444444444444444444443
%N Primes of the form 40*R_k + 3, where R_k is the repunit (A002275) of length k.
%C Primes of the form (4*10^k - 13)/9; the next term a(9) is a 183-digit number. - _Vincenzo Librandi_, Nov 16 2010
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/4/44443.htm#prime">Prime numbers of the form 44...443</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%t f[n_] := (4*10^n-13)/9; Select[f[Range[50]], PrimeQ] (* _Amiram Eldar_, Nov 23 2018 *)
%t Select[Table[FromDigits[PadLeft[{3},n,4]],{n,40}],PrimeQ] (* _Harvey P. Dale_, Dec 19 2022 *)
%Y Cf. A056661 (corresponding k), A173770.
%K nonn
%O 1,1
%A _Rick L. Shepherd_, Mar 26 2004
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