F(z) limit using formal definition

In summary, the problem involves using the formal limit definition to find the limit of f(z) = |z|^2 / z as z approaches 0. By considering the limit along the real and imaginary axes, we can see that the limit may be 0. To prove this, we can use the fact that |z| / |z| = 1 and manipulate the expression to show that it approaches 0 as z approaches 0. Therefore, we can choose epsilon = delta and justify this choice by showing that for any epsilon > 0, there exists a delta > 0 such that the absolute value of f(z) is less than epsilon whenever the absolute value of z is less than delta.
  • #1
Meggle
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Homework Statement


For each of the following choices of f(z) use the definition of a limit to obtain lim z-->0 f(z) or prove that the limit doesn't exist
(a) [tex]\frac{|z|^{2}}{z}[/tex]

Homework Equations


Formal limit definition

The Attempt at a Solution


f(z) = [tex]\frac{|z|^{2}}{z}[/tex]
f(z) = [tex]\frac{x^{2} + y^{2}}{x +iy}[/tex]
So if z=(x,0), f(z)=[tex]\frac{x^{2} + 0}{x +i0}[/tex] = x
Then f(z) --> 0 as (x,y) --> 0 along the real axis
And if z=(0,y), f(z)=[tex]\frac{0 + y^{2}}{0 +iy}[/tex] = [tex]\frac{y}{i}[/tex]
Then f(z) --> 0 as (x,y) --> 0 along the imaginary axis
So maybe lim z-->0 f(z)= 0
Suppose lim z-->0 f(z)= 0, then for each [tex]\epsilon[/tex] >0 there exists [tex]\delta[/tex] >0 such that 0 < |z - 0| < [tex]\delta[/tex] implies |f(z) - 0| < [tex]\epsilon[/tex]
...
Sooooo how do I figure delta out of that? I can't see how to simplify it or what to do next. It's due tomorrow, of course. :smile: And there's a (b) and a (c) , but think I could work them out if I could finish this one.

Also, can anyone tell me how to make the formulas update? I've changed all the SUP to curly brackets and carrots, but it won't seem to referesh. Edit never mind, seems they just don't show right on preview.
 
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  • #2
Here's a suggestion:
[tex]\left| \frac{z}{|z|} \right| = 1 [/tex]
So if you write
[tex] |f(z)| = \left| |z| \cdot \left( \frac{z}{|z|} \right)^{-1} \right| [/tex]
you can work in your delta.
 
  • #3
Epsilon= delta. Try to justify it.
 

FAQ: F(z) limit using formal definition

1. What is the formal definition of a limit?

The formal definition of a limit is the mathematical concept that describes the behavior of a function as the input approaches a certain value. It is written as "the limit of f(x) as x approaches a certain value is equal to L."

2. How is the formal definition of a limit used in calculus?

The formal definition of a limit is an important tool in calculus that allows us to analyze the behavior of functions, identify discontinuities, and determine the convergence or divergence of sequences and series.

3. What is the difference between a one-sided and two-sided limit?

A one-sided limit only considers the behavior of a function as the input approaches the certain value from one direction, while a two-sided limit considers the behavior from both directions.

4. How do you solve a limit using the formal definition?

To solve a limit using the formal definition, you must first rewrite the limit expression in the form of the definition. Then, you can use algebraic techniques and the properties of limits to simplify the expression and evaluate the limit.

5. What are the limitations of using the formal definition of a limit?

The formal definition of a limit can be time-consuming and challenging to use for more complex functions. It also does not guarantee that a limit exists, and it can be difficult to determine the exact value of the limit, only its existence.

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