Measure theory and Cantor function

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Homework Help Overview

The discussion revolves around the existence of a continuous, strictly increasing function on the interval [0, 1] that maps a set of positive measure onto a set of measure zero, specifically in the context of measure theory and the Cantor function.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning

Approaches and Questions Raised

  • Participants explore the properties of the Cantor function and its mapping characteristics. There are attempts to modify the Cantor function to achieve the desired mapping. Questions arise regarding the measure of the image set and the implications of continuity and monotonicity.

Discussion Status

Some participants have proposed modifications to the Cantor function, such as defining a new function based on it. There is ongoing exploration of the implications of these modifications on the measures of the sets involved. Multiple interpretations of the properties of the Cantor function and its modifications are being discussed.

Contextual Notes

Participants are considering the implications of the Cantor function's properties, including its continuity and the measure of its image. There is a focus on how to construct a function that meets the problem's requirements while addressing the constraints of measure theory.

sbashrawi
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Homework Statement


Show that there is a continuous , strictly increasing function on the interval [0, 1] that maps a set of positive measure onto a set of measure zero


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The Attempt at a Solution



I need to find a mapping to a countable set or cantor set but I couldn't construct such function
 
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TRY the Cantor function. Why doesn't it work? What might you do to modify it?
 
Cantor function is amapping from [0,1] onto [0,1] , I need a restriction so that its image has measure zero. But it is seemed to me that its image is countable am I right?
 
sbashrawi said:
Cantor function is amapping from [0,1] onto [0,1] , I need a restriction so that its image has measure zero. But it is seemed to me that its image is countable am I right?

It's image is [0,1]. That's not countable. Nor does it have measure zero. But those aren't the things you need. If C(x) is the Cantor function, it's not strictly monotone increasing, is it? Can you tell me why not? There's an easy way to modify it into a function from [0,1] to [0,2] that is strictly monotone increasing.
 
We have the function : h(x) = C(x) + x , where C(x) is the cantor function, the function h(x) is increasing function from [0,1] onto [0,2].
[0,1] = C union O : c = cantor set , O its complement.
m( C) = 0 , m(O) = 1 .
how can I modify this to get the required function?
 
sbashrawi said:
We have the function : h(x) = C(x) + x , where C(x) is the cantor function, the function h(x) is increasing function from [0,1] onto [0,2].
[0,1] = C union O : c = cantor set , O its complement.
m( C) = 0 , m(O) = 1 .
how can I modify this to get the required function?

That's already (almost) the function you want. m([0,2])=2. What are m(h(O)) and m(h(C))?
 
m( h(O)) = 1 and m(h(c))= 1 .
I think we can use the inverse function of h . it is contniuous , increasing
and hinv: [0,2] to [0,1]
and h inv(h(c)) = c which has measure zero i.e it maps a set with positive measure to a set of measure zero as required.

let f(x) = h(x) / 2 we get the required function: f: [0,1] to [0,1]
 
sbashrawi said:
m( h(O)) = 1 and m(h(c))= 1 .
I think we can use the inverse function of h . it is contniuous , increasing
and hinv: [0,2] to [0,1]
and h inv(h(c)) = c which has measure zero i.e it maps a set with positive measure to a set of measure zero as required.

let f(x) = h(x) / 2 we get the required function: f: [0,1] to [0,1]

That's it.
 

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