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Numbers n such that 8*10^n + 4*R_n - 3 is prime, where R_n = 11...1 is the repunit (A002275) of length n.
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%I #20 Jan 17 2019 13:44:07

%S 0,26,36,38,60,80,92,174,486,627,999,1614,4382,4898,8442,17262,17802,

%T 35427,40743

%N Numbers n such that 8*10^n + 4*R_n - 3 is prime, where R_n = 11...1 is the repunit (A002275) of length n.

%C Also numbers n such that (76*10^n-31)/9 is prime.

%C a(20) > 10^5. - _Robert Price_, Oct 20 2015

%H Makoto Kamada, <a href="https://stdkmd.net/nrr/8/84441.htm#prime">Prime numbers of the form 844...441</a>.

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

%F a(n) = A101066(n-1) + 1, for n>1.

%t Do[ If[ PrimeQ[(76*10^n - 31)/9], Print[n]], {n, 0, 10000}]

%o (PARI) for(n=0, 1e3, if(isprime((76*10^n-31)/9), print1(n", "))) \\ _Altug Alkan_, Oct 20 2015

%Y Cf. A002275, A101066.

%K more,nonn

%O 1,2

%A _Robert G. Wilson v_, Jan 19 2005

%E a(16)-a(19) from Kamada data by _Robert Price_, Dec 14 2010

%E Inserted a(1)=0 by _Robert Price_, Oct 20 2015