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%I #24 May 03 2024 14:07:52
%S 1,2,8,26,66,74,233,473,540,2774,64715,102492,141594
%N Numbers k such that 5*R_k - 2 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C Also numbers k such that (5*10^k-23)/9 is prime.
%C a(12) > 10^5. - _Robert Price_, Nov 13 2014
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/5/55553.htm#prime">Prime numbers of the form 55...553</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%F a(n) = A056685(n) + 1.
%t Do[ If[ PrimeQ[ 5(10^k - 1)/9 - 2], Print[k]], {k, 1, 5000}]
%Y Cf. A002275, A056685.
%K nonn
%O 1,2
%A _Robert G. Wilson v_, Oct 14 2004
%E a(11) from _Robert Price_, Nov 13 2014
%E 0 removed by _Georg Fischer_, Jan 03 2021
%E a(12)-a(13) from Kamada data by _Tyler Busby_, May 03 2024
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