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%I #14 Jul 08 2021 06:24:12
%S 2,277,2777,2777777777,2777777777777777,2777777777777777777,
%T 2777777777777777777777777777777777777,
%U 2777777777777777777777777777777777777777777777777777777777777777
%N Primes of the form 2*10^k + 7*R_k, where R_k is the repunit (A002275) of length k.
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/2/27777.htm#prime">Prime numbers of the form 277...77</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%t Select[FromDigits/@Table[PadRight[{2},n,7],{n,200}], PrimeQ] (* _Harvey P. Dale_, May 14 2011 *)
%Y Cf. A002275, A056702 (corresponding k).
%K nonn
%O 1,1
%A _Rick L. Shepherd_, Apr 17 2004
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