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%I #26 Jul 04 2021 22:09:20
%S 11,12,608
%N Numbers k such that 50*R_k + 1 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C Also numbers k such that (5*10^(k+1)-41)/9 is prime.
%C a(4) > 10^5. - _Robert Price_, Nov 16 2014
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/5/55551.htm#prime">Prime numbers of the form 55...551</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%F a(n) = A099415(n) - 1. - _Robert Price_, Nov 16 2014
%t Do[ If[ PrimeQ[50*(10^n - 1)/9 + 1], Print[n]], {n, 15000}]
%Y Cf. A002275, A099415.
%K hard,nonn,bref,more
%O 1,1
%A _Robert G. Wilson v_, Aug 10 2000
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