Sunday, 2 August 2015

inequality - How to solve $|z^2-1|



How to solve |z21|<|z|2 where z is a complex number? I have tried it both with cartesian and polar coordinates but did not get a solution.



I got that far: z=x+yi and then I got: ±x>(y2+0.51+4y2)0.5 but I don't know how to visualise that in the coordinate system.



Answer



That is equivalent to
|z21|<|z2|
This means that z2 is at a shorter distance from 1 than from 0. Then Re(z2)>1/2.



Now, write z=x+iy, thus, z2=(x2y2)+2xyi. The former inequality becomes
x2y2>12


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