Can a Subsequence of Measurable Functions Converge in L1?

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In summary, a subsequence of measurable functions can converge in L1, meaning that the sequence of functions converges in the L1-norm. The L1-norm is a measure of distance between functions in the space of integrable functions, defined as the integral of the absolute value of a function over its domain. Not all measurable functions are integrable, as a function is considered integrable if its L1-norm is finite. Convergence in L1 is a stronger form of convergence than pointwise convergence, with applications in mathematics and statistics including the study of Fourier series and the proof of the Central Limit Theorem. It also has applications in probability theory, defining the concept of convergence in distribution.
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Funky1981
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Homework Statement



Let fn be a sequence of measurable functions converges to f a.e. Is it possible to get a subsequence fnk of fn s.t. fn converges in L1 ?

2. The attempt at a solution
I have proved the converse statement is true and i guess the above statement is impossible but I fail to prove it.
 
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Hint: Look for a counterexample where ##f_n \rightarrow 0## pointwise but ##\|f_n\| = 1## for all ##n##.
 
  • #3
Thanks for your suggestion
 

FAQ: Can a Subsequence of Measurable Functions Converge in L1?

1. Can a subsequence of measurable functions converge in L1?

Yes, a subsequence of measurable functions can converge in L1. This means that the sequence of functions converges in the L1-norm, which is a measure of distance between functions in the space of integrable functions.

2. What is the L1-norm?

The L1-norm is a measure of distance between functions in the space of integrable functions. It is defined as the integral of the absolute value of a function over its domain.

3. Are all measurable functions integrable?

No, not all measurable functions are integrable. A function is considered integrable if its L1-norm is finite, meaning that it is possible to calculate the integral of the absolute value of the function over its domain.

4. How is convergence in L1 different from pointwise convergence?

Convergence in L1 is a stronger form of convergence than pointwise convergence. Pointwise convergence only requires that a sequence of functions approaches a limit at each point in the domain, while convergence in L1 requires that the L1-norm of the difference between the limit and the functions in the sequence approaches zero.

5. Are there any applications of convergence in L1?

Yes, there are several applications of convergence in L1 in mathematics and statistics. It is commonly used in the study of Fourier series and in the proof of the Central Limit Theorem. It also has applications in probability theory, where it is used to define the concept of convergence in distribution.

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