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Numbers k such that 8*10^k + 7*R_k - 4 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
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%I #19 May 04 2024 03:58:38

%S 1,7,36,99,325,1227,9261,9414,11115,25149,43212,95214

%N Numbers k such that 8*10^k + 7*R_k - 4 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

%C Also numbers k such that (79*10^k - 43)/9 is prime.

%H Makoto Kamada, <a href="https://stdkmd.net/nrr/8/87773.htm#prime">Prime numbers of the form 877...773</a>.

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

%F a(n) = A101077(n) + 1.

%t Do[ If[ PrimeQ[(79*10^n - 43)/9], Print[n]], {n, 0, 10000}]

%Y Cf. A002275, A101077.

%K more,nonn

%O 1,2

%A _Robert G. Wilson v_, Jan 19 2005

%E a(9)-a(11) from Kamada data by _Robert Price_, Dec 14 2010

%E a(12) from Kamada data by _Tyler Busby_, May 03 2024