%I A056706
%S A056706 1,3,13,25,72,108,375,393,589,973
%N A056706 Numbers n such that 4*10^n + 1*R_n is prime, where R_n = 11...1 is the 
               repunit (A002275) of length n.
%C A056706 Also numbers n such that (37*10^n-1)/9 is prime.
%H A056706 Makoto Kamada, <a href="http://homepage2.nifty.com/m_kamada/math/41111.htm">
               Factorizations of 411...11</a>
%H A056706 <a href="Sindx_Pri.html#Pri_rep">Index entries for primes involving repunits</
               a>
%t A056706 Do[ If[ PrimeQ[ 4*10^n + (10^n-1)/9], Print[n]], {n, 0, 10000}]
%Y A056706 Sequence in context: A146379 A056714 A120074 this_sequence A052454 A066947 
               A031011
%Y A056706 Adjacent sequences: A056703 A056704 A056705 this_sequence A056707 A056708 
               A056709
%K A056706 hard,nonn
%O A056706 1,2
%A A056706 Robert G. Wilson v (rgwv(AT)rgwv.com), Aug 10 2000