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%I #20 Jul 08 2021 03:11:04
%S 1,2,218,692,1805,2207,2873,59135
%N Numbers k such that 3*10^k + R_k + 6 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C Also numbers k such that (28*10^k + 53)/9 is prime.
%C All except 1, 2 and 218 only probably prime. No others less than 10000.
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/3/31117.htm#prime">Prime numbers of the form 311...117</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = A101826(n) + 1.
%e n = 1, 2 are members since 37 and 317 are primes.
%t Do[ If[ PrimeQ[(28*10^n+53)/9], Print[n]], {n, 0, 10000}]
%Y Cf. A100501, A100473, A101826.
%K nonn
%O 1,2
%A Julien Peter Benney (jpbenney(AT)ftml.net), Jan 15 2005
%E a(8) from Kamada data by _Robert Price_, Dec 13 2010
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