%N Prime numbers n such that 10^n+1 can be written in the form a^2+7*b^2.
%C These numbers of the form (10^n+1) have no prime factors congruent to 3 (mod 7) or 5 (mod 7) or 6 (mod 7) to an odd power. Also the power of 2 is not 1.
%H Samuel S. Wagstaff, Jr. <a href="http://homes.cerias.purdue.edu/~ssw/cun/index.html">The Cunningham Project</a>
%A _V. Raman_, Sep 08 2012