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Primes of the form 40*R_k + 9, where R_k is the repunit (A002275) of length k.
2

%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