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A093944 Primes of the form 9*10^k + 7*R_k, where R_k is the repunit (A002275) of length k. 1

%I #14 Jul 08 2021 05:31:24

%S 97,977,97777,97777777777777777777,97777777777777777777777777777,

%T 97777777777777777777777777777777777777777777777777777777777777777777777777

%N Primes of the form 9*10^k + 7*R_k, where R_k is the repunit (A002275) of length k.

%C The next term has 204 digits. - _Harvey P. Dale_, May 22 2014

%H Makoto Kamada, <a href="https://stdkmd.net/nrr/9/97777.htm#prime">Prime numbers of the form 977...77</a>.

%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>

%t Select[Table[FromDigits[PadRight[{9},n,7]],{n,250}],PrimeQ] (* _Harvey P. Dale_, May 22 2014 *)

%Y Cf. A002275, A056727 (corresponding k).

%K nonn

%O 1,1

%A _Rick L. Shepherd_, Apr 17 2004

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Last modified March 28 05:02 EDT 2024. Contains 371235 sequences. (Running on oeis4.)