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%I #19 Sep 08 2022 08:45:13
%S 7,47,4447,4444444447,44444444444444444447,44444444444444444444444447
%N Primes of the form 40*R_k + 7, where R_k is the repunit (A002275) of length k.
%C Primes of the form ((4*10^k - 31)/9) + 6. - _Vincenzo Librandi_, Dec 13 2011
%C The next term has 722 digits. - _Harvey P. Dale_, Jan 19 2020
%H Vincenzo Librandi, <a href="/A092480/b092480.txt">Table of n, a(n) for n = 1..7</a>
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/4/44447.htm#prime">Prime numbers of the form 44...447</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%t Select[Table[(((4*10^n-31)/ 9)+6),{n,1,800}],PrimeQ] (* _Vincenzo Librandi_, Dec 13 2011 *)
%t Select[Table[FromDigits[PadLeft[{7},n,4]],{n,30}],PrimeQ] (* _Harvey P. Dale_, Jan 19 2020 *)
%o (Magma) [a: n in [1..720] | IsPrime(a) where a is ((4*10^n-31) div 9)+6 ]; // _Vincenzo Librandi_, Dec 13 2011
%Y Cf. A056682 (corresponding k).
%K nonn
%O 1,1
%A _Rick L. Shepherd_, Apr 03 2004