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Primes of the form 80*R_k + 1, where R_k is the repunit (A002275) of length k.
2

%I #20 Jul 08 2021 06:20:38

%S 881,8888888888888888881,

%T 8888888888888888888888888888888888888888888888888888888888888888888888888888881

%N Primes of the form 80*R_k + 1, where R_k is the repunit (A002275) of length k.

%C Primes of the form 888...881.

%C The number of 8's in each term is given by the corresponding term of A056664 and so the first term too large to include above is 888...8881 (with 138 8's).

%C Primes of the form (8*10^k - 71)/9. - _Vincenzo Librandi_, Nov 16 2010

%H Makoto Kamada, <a href="https://stdkmd.net/nrr/8/88881.htm#prime">Prime numbers of the form 88...881</a>.

%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>

%t Select[Table[10 FromDigits[PadRight[{},n,8]]+1,{n,150}],PrimeQ] (* _Harvey P. Dale_, Aug 07 2019 *)

%Y Cf. A056664 (corresponding k).

%Y Cf. A051200, A004022, A093176, A093177, A089346, A093174, A092571.

%K bref,nonn

%O 1,1

%A _Rick L. Shepherd_, Mar 02 2004