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%I #11 Jul 08 2021 06:23:50
%S 7,17,1117,11117,11111117,11111111111111111111117,
%T 11111111111111111111111111117,
%U 1111111111111111111111111111111111111117
%N Primes of the form 10*R_k + 7, where R_k is the repunit (A002275) of length k.
%C The next term has 131 digits. - _Harvey P. Dale_, Jul 05 2021
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/1/11117.htm#prime">Prime numbers of the form 11...117</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%t Select[Table[FromDigits[PadLeft[{7},n,1]],{n,50}],PrimeQ] (* _Harvey P. Dale_, Jul 05 2021 *)
%Y Cf. A056655 (corresponding k).
%K nonn
%O 1,1
%A _Rick L. Shepherd_, Mar 23 2004
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