%I A056693
%S A056693 1,65,85,89,101,385,623,7783
%N A056693 Numbers n such that 70*R_n + 9 is prime, where R_n = 11...1 is the repunit 
               (A002275) of length n.
%C A056693 Also numbers n such that (7*10^(n+1)+11)/9 is prime.
%H A056693 <a href="Sindx_Pri.html#Pri_rep">Index entries for primes involving repunits</
               a>
%H A056693 Makoto Kamada, <a href="http://homepage2.nifty.com/m_kamada/math/77779.htm">
               Factorizations of 77...779</a>.
%t A056693 Do[ If[ PrimeQ[70*(10^n - 1)/9 + 9], Print[n]], {n, 0, 5000}]
%Y A056693 Sequence in context: A060877 A113688 A159758 this_sequence A164282 A025312 
               A024508
%Y A056693 Adjacent sequences: A056690 A056691 A056692 this_sequence A056694 A056695 
               A056696
%K A056693 hard,nonn
%O A056693 1,2
%A A056693 Robert G. Wilson v (rgwv(AT)rgwv.com), Aug 10 2000
%E A056693 More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008