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Primes of the form 7*10^k + R_k, where R_k is the repunit (A002275) of length k.
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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