# Linear fractional transformation

1. Apr 10, 2006

### sweetvirgogirl

sooo ...
i am kind of clueless about how to determine a linear fractional transformation for a circle that maps on to a line or vice versa ...

like i do *kinda* get how to map a circle on to a circle ... or a line on to a line ...

2. Apr 10, 2006

### Hurkyl

Staff Emeritus
Well, a line is just a circle that passes through the point at infinity! So...

3. May 3, 2006

### sweetvirgogirl

the way prof showed it in class ... he said pick three points on the circle and send it to three points on the line (or vice versa if we are trying to map a line to a cirle) ... but i am not sure how do i know what those three points are going to map to ....

on a circle |z| = 1, we could send i to infinity and -i to infinity and then the third point? maybe i am taking a wrong approach .... or maybe the prof didnt mean to tell us to take this approach and i misunderstood him

4. May 3, 2006

### shmoe

A linear fractional transformation sends only one point to infinity, you can't send two there. You have to take 3 distinct points on your circle and map them to 3 distinct points on your line.

5. May 3, 2006

### sweetvirgogirl

oops typo ... i meant we could send i to infinity and -i to - infinity and then the third point?

(-infinity and infinity are different, right?)

6. May 3, 2006

### AKG

No, infinity and -infinity are the same.

7. May 3, 2006

### sweetvirgogirl

woopsie ... let's restart then ...

I send i to infinity, -i to zero ... what about the third point?
it has to do something with fixing orientation, right? how does that work?

(like i know the third point can not be totally arbitrary like the first two points were, right?)

8. May 3, 2006

### shmoe

The third point just has to be on the 'target' line. If you're going for the real axis, you could take the third point to be 1. Have you seen the cross ratio?

Different choices of the third point on the real axis will get different transformations, but will still send your circle to the real axis.

9. May 3, 2006

### sweetvirgogirl

no we havent covered cross ratio in class yet.

so i *sorta* get how to send a circle to a line ... ...
the circle |z| = 1 and the line Re((1+i)w) = 0 ... so i sent 1 to infinity, -1 to 0 and i = -1 and i came up with i*(z+1)/(z-1) ...
the book has the answer as (1-i)*(z+1)/(z-1)
i wanna make sure my answer is right ...

also... to send a line to a circle ... i pick three arbitrary distinct points on the line and send them to three distinct points on the circle?

also ... if you have free time and you dont mind talking to a stranger over yahoo/aim ... please let me know ... thanks! :)

10. May 3, 2006

### shmoe

-1 is not on that line.

Yes.

I don't have any chat programs, sorry.

11. May 3, 2006

### sweetvirgogirl

how exactly do you determine that? coz the way i saw it ... -1 was on the line ... maybe i need some rest

12. May 3, 2006

### AKG

If -1 were on that line, then -1 would be one of the w such that Re((1+i)w) = 0, i.e. we would get:

Re((1+i)(-1)) = 0
Re(-1-i) = 0
-1 = 0

13. Jun 23, 2007

### qxred

There is no linear fractional function that maps a line on to a circle, or a circle on to a line, (unless the circle is a point), since linear fractional function preserves convexity. i.e., image of any convex set is convex , and the inverse image of any convex set is convex. See Stephen Boyd, Lieven Vandenberghe: Convex Optimization, Page 39

Last edited: Jun 23, 2007