Prove the function is continuous (topology)

Click For Summary
SUMMARY

The discussion focuses on proving the continuity of the function \( I: (X,d^*) \rightarrow (X,d) \), where \( I(f) = \int_{a}^{b} f(t) dt \) for continuous functions \( f: [a,b] \rightarrow \mathbb{R} \). The metric \( d^*(f,g) = \int_{a}^{b} |f(t) - g(t)| dt \) is used to establish the relationship between the continuity of \( I \) and the metrics \( d \) and \( d^* \). The key argument involves showing that for any \( \varepsilon > 0 \), there exists a \( \delta > 0 \) such that if \( d^*(f,g) < \delta \), then \( d(I(f), I(g)) < \varepsilon \). The discussion highlights the need for a counterexample to demonstrate the failure of the proposed inequality.

PREREQUISITES
  • Understanding of metric spaces and continuity definitions
  • Familiarity with integration of continuous functions
  • Knowledge of the properties of the \( L^1 \) norm
  • Experience with counterexamples in mathematical proofs
NEXT STEPS
  • Study the properties of the \( L^1 \) norm and its implications for continuity
  • Explore counterexamples in metric space continuity proofs
  • Learn about the Dominated Convergence Theorem and its applications
  • Investigate the relationship between different metrics on function spaces
USEFUL FOR

Mathematics students, particularly those studying real analysis and topology, as well as educators seeking to understand continuity in function spaces.

DotKite
Messages
81
Reaction score
1

Homework Statement


Let X be the set of continuous functions ## f:\left [ a,b \right ] \rightarrow \mathbb{R} ##.
Let d*(f,g) = ## \int_{a}^{b}\left | f(t) - g(t) \right | dt ## for f,g in X. For each f in X set,

## I(f) = \int_{a}^{b}f(t)dt ##

Prove that the function ## I ##: (X,d*) → (X,d) is continuous

Homework Equations



The definition of d is given as d(## a_{i},b_{i} ##) = max ## \left \{ \left | a_{i} - b_{i} \right | \right \} ##

Here is the definition of continuity I am working with

Let (X,d) and (Y,d') be metric spaces, and let a be in X. A function f : X→Y is said to be continuous at the point a in X if ## \varepsilon > 0 ## and there exists a ## \delta > 0 ## such that

d'(f(x),f(a)) < ## \varepsilon ##

whenever x is in X and

d(x,a) < ## \delta ##


The Attempt at a Solution



Let ## \varepsilon > 0 ##, there exists a ## \delta > 0 ## such that if d*(f,g) < ## \delta ##

then

d(I(f), I(g)) = max {|I(f) - I(g)|} = max ## \left \{ \left | I(f)-I(g) \right | \right \} ## = max ## \left \{ \left | \int_{a}^{b} f(t)dt - \int_{a}^{b}g(t) dt \right | \right \} ## = max ## \left \{ \left | \int_{a}^{b} f(t) - g(t) dt \right | \right \} ##...


So I was thinking that i should let ## \delta = \varepsilon ## and somehow show

max ## \left \{ \left | \int_{a}^{b} f(t) - g(t) dt \right | \right \} ## < ## \delta ##

I could show that by showing max ## \left \{ \left | \int_{a}^{b} f(t) - g(t) dt \right | \right \} ## < ## \int_{a}^{b}\left | f(t)-g(t) \right |dt ## because d(f,g) < ## \delta ##

But I don't think that inequality is true, and this is where I am stuck
 
Physics news on Phys.org
Find a counterexample if you think it is untrue.
 

Similar threads

  • · Replies 14 ·
Replies
14
Views
2K
  • · Replies 11 ·
Replies
11
Views
3K
Replies
9
Views
2K
Replies
2
Views
2K
  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 14 ·
Replies
14
Views
2K
Replies
6
Views
3K
  • · Replies 105 ·
4
Replies
105
Views
11K
  • · Replies 15 ·
Replies
15
Views
2K
  • · Replies 6 ·
Replies
6
Views
2K