Friday, 15 July 2016

real analysis - Verify if n dimensional spheres intersect



Consider x1,,xn vectors lying in Rn. Also, consider positive numbers α1,,αn. Consider n spheres with center as xi and radii αi. How do I check the set of solutions to the set of equations
||xxi||2=αi , i
is non-empty. Is this problem studied in literature?


Answer



The question is well-studied. For example, you can read this.



One of the methods works like this:
Rewrite your equation as
2xix=x2+x2iα2i.
Find x in form x=ru+v, where r=x2, u is the solution of 2xiu=1, v is the solution of 2xiv=x2iα2i. Then
r=x2=(ru+v)2=r2u2+2uvr+v2
is a quadratic equation on r. If this equation has roots, then your n spheres have a common point.


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