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%I #16 Jul 08 2021 06:24:32
%S 7,557,5557,555557,5555555557,555555555555557,5555555555555555555557,
%T 5555555555555555555555555555555555555555555555555555555555555555555555555555555555555557
%N Primes of the form 50*R_k + 7, where R_k is the repunit (A002275) of length k.
%C Primes of the form (5*10^k + 13)/9. - _Vincenzo Librandi_, Nov 16 2010
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/5/55557.htm#prime">Prime numbers of the form 55...557</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%Y Cf. A056686 (corresponding k).
%K nonn
%O 1,1
%A _Rick L. Shepherd_, Mar 26 2004