Tuesday, 3 June 2014

Find $16$ complex numbers $x + yi, x,yin mathbb{Z}$ with norm $N(x + yi) =65$



I took some integer value for $x,y$, as $x=3, y=2$ with norm $N = \sqrt{13}$. But, no it is not. I hoped that by finding a relation between current value of $x,y, N$ to the desired one; would be able to get the desired $x',y'$. But could not find any such relation, as :
So, tried to square the values of $x,y$ to get $x'=x^2=9, y'=y^2=4$; as hoped that it will have norm $13$.



Definitely, the reason lies in adding $2x'\cdot y'$ also, whose square root need be split in $x',y'$.



To check this idea, $x'^2= x^4 = 81; y'^2 = y^4 = 16; 2x'y'= 72$ and $x'^2 + y'^2 + 2x'\cdot y' = 81+16+72 = 169$.




The square root of $72$ is not an integer, so cannot go further.



So, for getting a norm of $65$ cannot proceed from norm of $\sqrt{13}$.



Could not get any other way out. Please tell a logical approach to generate all such pairs.






Addendum Sorry, for flawed defn. of norm. Total question is created out of that wrong definition.



Answer



$$65=5\times 13=|2+i|^2|3+2i|^2=|(2+i)(3+2i)|^2
=|4+7i|^2$$
$$65=5\times 13=|2+i|^2|3-2i|^2=|(2+i)(3-2i)|^2
=|8-i|^2$$
etc.


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