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%I
%S 1,65,85,89,101,385,623,7783
%N Numbers n such that 70*R_n + 9 is prime, where R_n = 11...1 is the repunit (A002275) of length n.
%C Also numbers n such that (7*10^(n+1)+11)/9 is prime.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%H Makoto Kamada, <a href="http://homepage2.nifty.com/m_kamada/math/77779.htm">Factorizations of 77...779</a>.
%t Do[ If[ PrimeQ[70*(10^n - 1)/9 + 9], Print[n]], {n, 0, 5000}]
%K hard,nonn
%O 1,2
%A _Robert G. Wilson v_, Aug 10 2000
%E More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
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