%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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