# complex inequalities

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• November 26th 2011, 08:03 AM
minicooper58
complex inequalities
if |z|=3 is a circle, how do you know if the region outside the circle is
|z|<3

Many thanks
• November 26th 2011, 08:12 AM
Plato
Re: complex inequalities
Quote:

Originally Posted by minicooper58
if |z|=3 is a circle, how do you know if the region outside the circle is |z|<3 .

$|z-w|$ is the distance between $z~\&~w$.

Thus $\{z:|z|=|z-0|=3\}$ is a circle centered at the origin with radius 3.

BUT the exterior of that circle is $\{z:|z|=|z-0|>3\}$, not it is not $<$.
• November 26th 2011, 08:27 AM
minicooper58
2 Attachment(s)
Re: complex inequalities
Hi , the question and answer is in the attachments . sorry i didn't fully explain the transformation part . I don't understand how the region is outside the circle for (b) .

Many thanks
• November 26th 2011, 12:16 PM
HallsofIvy
Re: complex inequalities
That's not at all the question you originally asked! The set |z|< 3 is the inside of the circle given by |z|= 3. But the function $w= \frac{z}{z+ 1}$ maps the circle |z|= 3, with center at 0 and radius 3, into the circle with center at $\frac{9}{8}$ and radius $\frac{3}{8}$. It also clearly maps 0 to 0. 0 is inside |z|< 3 but outside the circle with center $\frac{9}{8}$ and radius $\frac{3}{8}$. Therefore, the interior of the first circle is mapped to the exterior of the second circle.
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