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%I #15 Jul 08 2021 06:20:17
%S 7,71,71111111,
%T 71111111111111111111111111111111111111111111111111111111
%N Primes of the form 7*10^k + R_k, where R_k is the repunit (A002275) of length k.
%C No further terms up to 2000 digits. - _Harvey P. Dale_, Mar 06 2012
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/7/71111.htm#prime">Prime numbers of the form 711...11</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%t Select[Table[FromDigits[PadRight[{7},n,1]],{n,2000}],PrimeQ] (* _Harvey P. Dale_, Mar 06 2012 *)
%Y Cf. A002275, A056719 (corresponding k).
%K nonn
%O 1,1
%A _Rick L. Shepherd_, Apr 06 2004