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A216527 Odd numbers n such that (10^n-1)/9 can be written in the form a^2+7*b^2. 0

%I

%S 1,7,13,19,23,37,47,49,53,59,71,73,83,91,101,103,127,133,149,151,161,

%T 191,211,259,266,317,329

%N Odd 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

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Last modified June 16 21:49 EDT 2021. Contains 345080 sequences. (Running on oeis4.)