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Numbers n such that (10^n-1)/9 can be written in the form a^2+7*b^2.
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%I #5 Sep 08 2012 15:25:17

%S 1,2,7,13,14,19,23,37,38,47,49,53,59,71,73,83,91,98,101,103,127,133,

%T 149,151,161,191,211,259,266,317,329

%N 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>

%K nonn

%O 1,2

%A _V. Raman_, Sep 08 2012