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%I #20 Jul 08 2021 06:20:35
%S 449,44449,444449,444444444444444444444444444444444444444444444449,
%T 444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444444449
%N Primes of the form 40*R_k + 9, where R_k is the repunit (A002275) of length k.
%C Primes of the form (4*10^k + 41)/9. - _Vincenzo Librandi_, Nov 16 2010
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/4/44449.htm#prime">Prime numbers of the form 44...449</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%t Select[Table[FromDigits[PadLeft[{9},n,4]],{n,120}],PrimeQ] (* _Harvey P. Dale_, May 21 2011 *)
%Y Cf. A056683 (corresponding k).
%K nonn
%O 1,1
%A _Rick L. Shepherd_, Mar 28 2004
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