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%I #10 Jun 21 2022 04:32:00
%S 3,6,7,13,19,21,31,42,49,61,67,91,97,103,109,127,133,147,199,217,229,
%T 241,271,294,343,349,427,457,469,499,523,589,601,607,637,673,679,721,
%U 733,751,763,823,877,889,907,931,997,1029
%N Numbers k such that 2^k-1 can be written in the form a^2 + 7*b^2.
%C These Mersenne numbers of the form 2^k-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>
%K nonn
%O 1,1
%A _V. Raman_, Sep 08 2012