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%I #14 Jul 08 2021 05:50:42
%S 47,47777,47777777777777,47777777777777777777777777,
%T 4777777777777777777777777777777777777
%N Primes of the form 4*10^k + 7*R_k, where R_k is the repunit (A002275) of length k.
%C The next term has 358 digits. - _Harvey P. Dale_, Nov 13 2013
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/4/47777.htm#prime">Prime numbers of the form 477...77</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%t Select[Table[FromDigits[PadRight[{4},n,7]],{n,500}],PrimeQ] (* _Harvey P. Dale_, Nov 13 2013 *)
%Y Cf. A002275, A056708 (corresponding k).
%K nonn
%O 1,1
%A _Rick L. Shepherd_, Apr 17 2004
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