What is the condition for a function to be integrable with improper integral?

In summary, for Riemann integral, a countable number of points where the function is undefined does not affect its integrability. However, for Lebesgue integral, the function can be undefined on a set of measure 0 and still be integrable. An even weaker condition is that the set is a subset of a measurable set of measure 0. For Riemann integrals, it is sufficient that the countable set of points where the function is undefined is not dense in any interval of the real numbers.
  • #1
LHeiner
8
0
Hello

i stumbled over a question and I'm not sure how to proof/ solve hat:

At how many points a function can not be defined to be nevertheless integrable (improper integral)?

Thx in advance!
 
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  • #2
If you are talking about Riemann integral, a countable number of points. However in many cases there could be a problem, since the set of rationals is countable.

For Lebesgue integral, a set of measure 0. This is a much better criterion.
 
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  • #3
Interestingly for lebesgue integrals a function can be left undefined on an uncountable subset of R and still be integrable.

An even weaker condition is weaker for lebesgue integrals though, the set need only be a subset of a measurable set of measure 0.

For riemann integrals I think it is sufficient that the countable set of points where the function is undefined is not dense in any interval (a,b) of R.
 

1. What is an improper integral?

An improper integral is an integral where one or both of the limits of integration are infinite or where the integrand function is not defined at one or more points within the interval of integration.

2. How do you determine if an improper integral converges or diverges?

To determine the convergence or divergence of an improper integral, you need to evaluate it using a limit. If the limit exists and is a finite number, then the integral converges. If the limit does not exist or is infinite, then the integral diverges.

3. Can an improper integral have a finite value even if it diverges?

Yes, it is possible for an improper integral to have a finite value even if it diverges. This can happen if the integral has a removable discontinuity or if the integral has a jump discontinuity at one or both of the limits of integration.

4. How is the comparison test used to evaluate improper integrals?

The comparison test is used to determine the convergence or divergence of an improper integral by comparing it to another integral whose convergence or divergence is already known. If the known integral converges and the given integral is greater than or equal to it, then the given integral also converges. If the known integral diverges and the given integral is less than or equal to it, then the given integral also diverges.

5. Are there any special techniques for evaluating improper integrals?

Yes, there are several techniques for evaluating improper integrals, including the limit comparison test, the ratio test, and the Cauchy condensation test. Additionally, some integrals can be evaluated using integration by parts or substitution. It is important to carefully consider the limits of integration and the properties of the integrand when using these techniques.

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