A216528 - OEIS

A216528

Even numbers n such that (10^n-1)/9 can be written in the form a^2+7*b^2.

%I #5 Sep 11 2012 15:34:38

%S 2,14,38,98

%N Even 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 1,1

%A _V. Raman_, Sep 08 2012