Simple question about Lebesgue and Borel sigma algebra

In summary, Lebesgue and Borel sigma algebras are different types of mathematical structures used to measure sets. The main difference between them is the types of sets they can measure, with Lebesgue sigma algebra being able to measure more sets than Borel sigma algebra. Borel sigma algebra is a subset of Lebesgue sigma algebra, with Lebesgue and Borel measures being defined on their respective sigma algebras. These sigma algebras are important in measure theory, as they provide a way to measure sets in various areas of mathematics. An example of a set that is measurable by Lebesgue sigma algebra but not by Borel sigma algebra is the Cantor set.
  • #1
quasar987
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If N is of null Lebesgue measure. Can we find a Borel set B of null measure such that N is entirely contained in B?
 
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  • #2
The empty set!
 
  • #3
it's the other way around. we want B borel such that N is contained in B
 
  • #4
bump!
 
  • #5
quasar987 said:
it's the other way around. we want B borel such that N is contained in B
Oops!

This is also true. It follows from the fact that the Lebesgue measure is regular.
 

Related to Simple question about Lebesgue and Borel sigma algebra

1. What is the difference between Lebesgue and Borel sigma algebra?

Lebesgue and Borel sigma algebras are both types of mathematical structures used to measure sets. The main difference between them is the types of sets they can measure. Lebesgue sigma algebra can measure more sets than Borel sigma algebra, as it includes all the sets that Borel sigma algebra can measure and some additional sets as well.

2. How are Lebesgue and Borel sigma algebras related?

Borel sigma algebra is a subset of Lebesgue sigma algebra, meaning that all sets measurable by Borel sigma algebra are also measurable by Lebesgue sigma algebra. In other words, Borel sigma algebra is a special case of Lebesgue sigma algebra.

3. What is the importance of Lebesgue and Borel sigma algebras in measure theory?

Lebesgue and Borel sigma algebras are fundamental concepts in measure theory, which is a branch of mathematics that deals with the concept of measuring sets. These sigma algebras provide a way to measure various types of sets, which is crucial in many areas of mathematics, such as probability theory and analysis.

4. How are Lebesgue and Borel measures related to their respective sigma algebras?

The Lebesgue measure is a measure defined on the Lebesgue sigma algebra, while the Borel measure is a measure defined on the Borel sigma algebra. Both measures assign a non-negative value to each set in their respective sigma algebra, with the Lebesgue measure being more comprehensive.

5. Can you give an example of a set that is measurable by Lebesgue sigma algebra but not by Borel sigma algebra?

Yes, the Cantor set is an example of a set that is measurable by Lebesgue sigma algebra but not by Borel sigma algebra. The Cantor set is a fractal set with a complicated structure, and it is not a Borel set, meaning it cannot be measured by Borel sigma algebra. However, it is measurable by Lebesgue sigma algebra, as it is a subset of the real numbers and can be defined using intervals, which are measurable by Lebesgue sigma algebra.

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