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%I #9 Aug 19 2015 18:31:24
%S 2,7,13,19,23,37,47,53,59,71,73,83,101,103,127,149,151,191,211,317
%N Prime numbers n such that (10^n-1)/9 can be written in the form a^2+7*b^2.
%C These repunit numbers of the form (10^n-1)/9 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>
%t Select[Prime[Range[70]],Length[Solve[a^2+7b^2==(10^#-1)/9,{a,b}, Integers]] > 0&] (* _Harvey P. Dale_, Aug 19 2015 *)
%K nonn
%O 1,1
%A _V. Raman_, Sep 08 2012
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