Proving Entire Function f(z) and its Image is All of C | Homework Help

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In summary, if f is an entire function and lim z->inf. f(z)=inf, then the image of f is all of C. This can be shown by considering an arbitrary value w in C and showing that there is a z such that f(z)=w. By considering the function g(z)=1/(f(z)-w), it can be shown that g(z) is analytic everywhere on C and approaches 0 as z approaches infinity. By the maximum modulus principle, g(z) must be constant, which implies that f(z) is also constant. This is a contradiction, thus showing that f(z) must attain all values in C.
  • #1
g1990
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Homework Statement


suppose that f is an entire function and that lim z->inf. f(z)=inf.
show that the image of f is all of C


Homework Equations


I know that f(z) must have isolated values


The Attempt at a Solution


I think the approach should be to pick an arbitrary value w in C and show that there is a z such that f(z)=w... but I don't know where to go from there
 
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  • #2
Suppose f(z) is never equal to w. Then what kinds of things can you say about g(z)=1/(f(z)-w)?
 
  • #3
lets see, if f(z) never equals w, then g(z) is analytic everywhere on C. Then, as z approaches infinity, g(z) approaches 0 because f(z) approaches infinity. I'm not sure why this is a contradiction.
 
  • #4
g1990 said:
lets see, if f(z) never equals w, then g(z) is analytic everywhere on C. Then, as z approaches infinity, g(z) approaches 0 because f(z) approaches infinity. I'm not sure why this is a contradiction.

Think about whether g(z) might be bounded.
 
  • #5
Lets see... the modulus of f is definitely bounded from below by 0. If I could get it to be bounded from above that would be a contradiction b/c of the maximum modulus principle. I'm not sure how to do that though. You could say that if z if large, f is above a certain value so that g is below a certain value. Then, f has a minimum value on the remaining compact set, so that g has a maximum on that compact set. Then the max of the two would be the bound for g. Is that right?
 
  • #6
g1990 said:
Lets see... the modulus of f is definitely bounded from below by 0. If I could get it to be bounded from above that would be a contradiction b/c of the maximum modulus principle. I'm not sure how to do that though. You could say that if z if large, f is above a certain value so that g is below a certain value. Then, f has a minimum value on the remaining compact set, so that g has a maximum on that compact set. Then the max of the two would be the bound for g. Is that right?

You are garbling that a bit. But you've got the basic idea right. Yes, it's compactness and |f(z)|->infinity. Try and state it a little more carefully and then I'll believe you.
 
  • #7
Alright, so pick an N. Then, when |z|>M for some value M, |f(z)|>N. Thus, when |z|>M, |g(z)|<1/|N-w|. When |z|<=N, then |g(z)| attains a maximum (on the boundary) because g is continuous and the disc |z|<=N is compact. Call this maximum P. Then, g(z) is bounded by max(P, 1/|N-w|). But, g is entire, so by the max-modulus principle g is constant. Then, f must also be constant because w is fixed. This is a contradiction. Does that sound better?
 
  • #8
Does |f(z)|>N really imply that 1/|f(z)-w|<1/|N-w|? Suppose N=1, f(z)=1.01*i and w=i. You are just being really sloppy. Shouldn't your choice of N somehow involve |w|?
 
  • #9
Lets see, if |f|>N, then |g|=1/|f-w|. By the triangle inequality, |f-w|>||f|-|w||>||N|-|w||
I could let N=S|w|, so that then |f-w|>||N|-|w||=||S||w|-|w||=|S-1|
Then, |g(z)|=1/|f-w|<1/(S-1). I don't get why having |w| in the bound of g(z) is a problem.
 
  • #10
Oh, maybe I need that N>|w|?
 
  • #11
g1990 said:
Oh, maybe I need that N>|w|?

N>2|w| will do it, won't it? Then |g(z)|<1/|w|.
 
  • #12
Okay, so if I let N>2|w|, then my argument will work?
 
  • #13
g1990 said:
Okay, so if I let N>2|w|, then my argument will work?

Sure.
 

1. How do I prove that a function f(z) is entire?

To prove that a function f(z) is entire, you need to show that it is analytic at every point in the complex plane. This means that the function must have a derivative at every point and that this derivative must also be continuous. Additionally, you need to show that the function is defined and continuous at every point.

2. What is an entire function?

An entire function is a complex function that is defined and analytic at every point in the complex plane. This means that the function has a derivative at every point and that this derivative is also continuous. In other words, an entire function is a complex function that is well-behaved and defined at every point in the complex plane.

3. How do I prove that the image of an entire function f(z) is all of C?

To prove that the image of an entire function f(z) is all of C (the complex plane), you need to show that for every point z in C, there is a corresponding point w in the image of f(z). This means that the function is surjective, or onto. Additionally, you need to show that the function is defined and continuous at every point in the complex plane.

4. What techniques can I use to prove that a function is entire?

There are several techniques that can be used to prove that a function is entire. Some common techniques include using the Cauchy-Riemann equations, the Cauchy integral formula, the power series expansion, and the identity theorem. It is important to carefully choose which technique to use based on the specific function and problem at hand.

5. Can a function be entire but not analytic?

No, a function cannot be entire but not analytic. The definition of an entire function includes the requirement that the function is analytic at every point in the complex plane. Therefore, if a function is entire, it must also be analytic at every point.

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