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Numbers n such that 2*10^n + 8*R_n + 1 is prime, where R_n = 11...1 is the repunit (A002275) of length n.
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%I #16 Jan 17 2019 13:44:07

%S 0,1,7,8,11,14,19,32,41,53,61,133,157,239,1237,30254,34423,139289

%N Numbers n such that 2*10^n + 8*R_n + 1 is prime, where R_n = 11...1 is the repunit (A002275) of length n.

%C Also numbers n such that (26*10^n+1)/9 is prime.

%H Makoto Kamada, <a href="https://stdkmd.net/nrr/2/28889.htm#prime">Prime numbers of the form 288...889</a>.

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

%F a(n) = A101972(n-1) + 1.

%t Do[ If[ PrimeQ[(26*10^n + 1)/9], Print[n]], {n, 0, 10000}]

%Y Cf. A002275, A101972.

%K more,nonn

%O 1,3

%A _Robert G. Wilson v_, Dec 17 2004

%E a(16)-a(18) from Kamada data by _Robert Price_, Dec 14 2010