F non-measurable but |f| and f^2 are

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In summary, the solution involves finding a function f in a measurable space such that both its absolute value and square are measurable. The explanation involves understanding that for all subsets S of the real numbers, the inverse images of |f| and f^2 are either the set X or the empty set, depending on whether or not S contains the number 1.
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diddy_kaufen
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Homework Statement



Find a function in a measurable space that is non-measurable, but |f| and f2 are measurable.

Homework Equations



None.

The Attempt at a Solution



I am trying to understand the following answer to the problem:

MMx7fFs.png

(source: http://math.stackexchange.com/a/1233792/413398)

I do not understand why for all subsets [itex]S \subseteq \mathbb{R}[/itex], we have $$(|f|)^{-1}(S)=(f^2)^{-1}(S)=X \text{ or } \varnothing$$
 
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Hi,

##f^2## or ##abs(f)## both send ##a## and ##b## to ##1##. So ##1## has ##a## and ##b## as antecedents, and any other real has no antecedent. So if ##S## contains ##1## it has ##X=\{a,b\}## as reciprocal image, and the empty set if it doesn't contain it.
 
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What does it mean for a function to be non-measurable?

A non-measurable function is one that cannot be assigned a definite value for every point in its domain. This typically occurs when the function is discontinuous or has an infinite number of discontinuities.

Why is it important to distinguish between non-measurable functions and functions with non-measurable values?

It is important to make this distinction because a function with non-measurable values may still be measurable, while a non-measurable function cannot be measured at all. This can have implications for the validity of mathematical proofs and theorems.

What is the difference between |f| and f^2 for a non-measurable function?

The absolute value of a non-measurable function may still be measurable, while the function itself is not. On the other hand, raising a non-measurable function to the power of 2 will also result in a non-measurable function.

Can a non-measurable function have measurable properties?

Yes, a non-measurable function may still have measurable properties such as continuity, integrability, and differentiability. However, these properties may not hold for all points in the function's domain.

What are some examples of non-measurable functions?

One example is the Dirichlet function, which takes on the value of 1 for rational numbers and 0 for irrational numbers. Another example is the Weierstrass function, which is continuous everywhere but differentiable nowhere.

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