x Turn on thread page Beta
 You are Here: Home >< Maths

# COMPLEX ANALYSIS Finding the inverse of the function g(z) = Log(z-i) and the domain. watch

1. I have a problem with showing that this complex function is one-one. Could someone help me please?

Show that the function g(z) has an inverse, and find its domain and rule.

g(z) = Log ( z-i )

I know the C - { i } is the domain of g, but I am unsure of the codomain. I know g^-1 will have codomain C-{i}.

I have let w = g(z) so that the rule for g^-1 = e^w + i which I am happy with. My problem is showing now that g^-1 is one-one, so that the inverse exists, and finding the codomain of g.
2. (Original post by AJ2357)
I have a problem with showing that this complex function is one-one. Could someone help me please?

Show that the function g(z) has an inverse, and find its domain and rule.

g(z) = Log ( z-i )

I know the C - { i } is the domain of g, but I am unsure of the codomain. I know g^-1 will have codomain C-{i}.

I have let w = g(z) so that the rule for g^-1 = e^w + i which I am happy with. My problem is showing now that g^-1 is one-one, so that the inverse exists, and finding the codomain of g.
I am bumping it for you because i want to see the answer
3. (Original post by AJ2357)
I have a problem with showing that this complex function is one-one. Could someone help me please?

Show that the function g(z) has an inverse, and find its domain and rule.

g(z) = Log ( z-i )

I know the C - { i } is the domain of g, but I am unsure of the codomain. I know g^-1 will have codomain C-{i}.

I have let w = g(z) so that the rule for g^-1 = e^w + i which I am happy with. My problem is showing now that g^-1 is one-one, so that the inverse exists, and finding the codomain of g.
what, *exactly* does Log mean here?
4. Hi Log is the complex logarithm defined as:

For z in the set C - {0} the principal logarithm of z is

Log z = ln|z| + i Arg|z|

It is the inverse of the function f(z) = e^z such that z is in the set {x+iy:-pi<y<=pi}

I have got this so far

Let g(z) = Log(z - i) so the domain of g is C - {i}. Now let C - {i}= A, then

g(A) = {Log(z-i) z: z is in the set C - {i} }

g(A) = {w=Log(z-i) z: z is in the set C - {i} }

g(A) = {w= ln|z-i| + i Arg(z-i) z: z is in the set C - {i} }

Now this is where I can't SEE the answer to what the codomain is. I think that w needs to be in the set C minus anything that makes the argument of w zero, but this could be incorrect!
5. (Original post by AJ2357)
Hi Log is the complex logarithm defined as:

For z in the set C - {0} the principal logarithm of z is

Log z = ln|z| + i Arg|z|

It is the inverse of the function f(z) = e^z such that z is in the set {x+iy:-pi<y<=pi}

I have got this so far

Let g(z) = Log(z - i) so the domain of g is C - {i}. Now let C - {i}= A, then

g(A) = {Log(z-i) z: z is in the set C - {i} }

g(A) = {w=Log(z-i) z: z is in the set C - {i} }

g(A) = {w= ln|z-i| + i Arg(z-i) z: z is in the set C - {i} }

Now this is where I can't SEE the answer to what the codomain is. I think that w needs to be in the set C minus anything that makes the argument of w zero, but this could be incorrect!
Log(z-i) is just going to take the same set of values on its domain as Log(z) does on its domain.

So if Log(a) = b then Log(z-i) takes the same value at a+i.
6. (Original post by AJ2357)
..
I'm not familiar with the term codomain (and the wiki page defn doesn't seem to me like it's what you're after(*)), but as RichE says, the range of values taken by Log(z-i) is going to be identical to the range taken by Log(z), so I would expect Log(z-i) to have the same codomain as Log(z).

(*) As defined in the wiiki, it's just the destination set when you say "f(z) is a function from X to Y". Since in this context it seems there are various possible choices for Y (some larger than others) it didn't really seem to fit what you're after.
7. (Original post by DFranklin)
I'm not familiar with the term codomain (and the wiki page defn doesn't seem to me like it's what you're after(*)), but as RichE says, the range of values taken by Log(z-i) is going to be identical to the range taken by Log(z), so I would expect Log(z-i) to have the same codomain as Log(z).

(*) As defined in the wiiki, it's just the destination set when you say "f(z) is a function from X to Y". Since in this context it seems there are various possible choices for Y (some larger than others) it didn't really seem to fit what you're after.
Thanks for your replies both. That makes it clearer now, but I'm not sure what the range of Log(z) is? This is where I keep going round in circles. Is it that the principal Logz has range -pi<Im(w)<=pi, and then I can say that Log(z-i) will have the same range?
8. (Original post by AJ2357)
Thanks for your replies both. That makes it clearer now, but I'm not sure what the range of Log(z) is? This is where I keep going round in circles. Is it that the principal Logz has range -pi<Im(w)<=pi, and then I can say that Log(z-i) will have the same range?
Yes. (Because for any "input" z you put into Log z, if you choose w = z+i as the "input" to Log(w-i), you're obviosuly going to get the same result.
9. (Original post by DFranklin)
Yes. (Because for any "input" z you put into Log z, if you choose w = z+i as the "input" to Log(w-i), you're obviosuly going to get the same result.
It is all very clear now. I really appreciate your help. Thank you!
10. (Original post by AJ2357)
It is all very clear now. I really appreciate your help. Thank you!
No problem. It was really nice to see someone actually answer fully when asked "what does Log mean here", as opposed to "it's log, innit? Why are you even trying to answer this if you don't know what log is?" (Such responses have happened...)

[I was fairly sure by use of the capital L that you meant what you meant, but since the answer would be quite different if you were allowing log to be multi-valued I needed to check].
11. My fault entirely by being less rigorous than necessary. It's these twists about not being specific enough that makes the topic very enjoyable, but also quite frustrating!
12. (Original post by AJ2357)
Let g(z) = Log(z - i) so the domain of g is C - {i}. Now let C - {i}= A, then

g(A) = {Log(z-i) z: z is in the set C - {i} }

g(A) = {w=Log(z-i) z: z is in the set C - {i} }

g(A) = {w= ln|z-i| + i Arg(z-i) z: z is in the set C - {i} }

Now this is where I can't SEE the answer to what the codomain is. I think that w needs to be in the set C minus anything that makes the argument of w zero, but this could be incorrect!
It seems to me that you could formalise this a bit as follows:

Let with

Let then

so with

Or have I missed the point?
13. (Original post by atsruser)
It seems to me that you could formalise this a bit as follows:

Let with

Let then

so with

Or have I missed the point?
I feel this misses the point, yes. Range of Log z is given and obviously range of Log(z-i) is going to be the same.
14. (Original post by DFranklin)
I feel this misses the point, yes. Range of Log z is given and obviously range of Log(z-i) is going to be the same.
[This is my third attempt at a sensible answer - I'm confusing myself ..]

He wants to know what set corresponds to , if I'm understanding this right. And now we can see trivially that corresponds to the image of the principal logarithm.

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: October 27, 2015
Today on TSR

### Complete university guide 2019 rankings

Find out the top ten here

Poll
Useful resources

### Maths Forum posting guidelines

Not sure where to post? Read the updated guidelines here

### How to use LaTex

Writing equations the easy way

### Study habits of A* students

Top tips from students who have already aced their exams