Is f(x) = 1 if x is rational, 0 if x is irrational Riemann integrable on [0,1]?

In summary, the given function f is Riemann integrable on the interval [0,1] due to the fact that for any partition, the supremum of the upper sum is 1 and the infimum of the lower sum is 0, while the width of the interval approaches 0 as the mesh of the partition decreases. This shows that the function is indeed integrable on the given interval with an integral value of 0.
  • #1
missavvy
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0

Homework Statement



Let A={1/n, n =natural number}
f: [0,1] -> Reals
f(x) = {1, x in E, 0 otherwise
Prove f is riemann integrable on [0,1]

Homework Equations





The Attempt at a Solution



Not quite sure, but I think supf = 1 and inf f= 0 no matter what partition you take, then
Spf - spf = 1
so it is not r-integrable..?
(obv i skipped lots of steps but I am not sure if it is actually r-int or not, i said its not)
 
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  • #2
That's not right; consider this:
You have a function that is 1 for x=1, but 0 everywhere else. You integrate in an interval including x=1.
The value of the function in the upper sum in the part of the partition containing x=1 is always 1, while the lower is 0. But the width of the interval in the partition containing x=1 diminishes to 0 as the mesh of the partition approaches 0. So the function is integrable, and the integral on every interval is 0.

This should reveal the mistake you were making. Can you apply this kind of reasoning to your example?
 

Related to Is f(x) = 1 if x is rational, 0 if x is irrational Riemann integrable on [0,1]?

What is Riemann integrability?

Riemann integrability is a mathematical concept that describes the ability to calculate the area under a curve using a specific method called the Riemann integral. It is used to find the exact value of an integral, which is the limit of a sum of infinitely many small rectangles under a curve.

How is Riemann integrability different from other types of integration?

Riemann integrability differs from other types of integration, such as Lebesgue integration, in the way it approximates the area under a curve. While Lebesgue integration partitions the domain into smaller subsets, Riemann integration divides the domain into equal intervals and uses the value of the function at the endpoints of each interval to calculate the area.

What are the conditions for a function to be Riemann integrable?

A function must be continuous on a closed interval and bounded in order to be Riemann integrable. This means that the function must have a defined value at every point within the interval and its values must not exceed a certain limit.

Why is Riemann integrability important in mathematics?

Riemann integrability is important in mathematics because it provides a way to calculate the exact value of an integral, which is a fundamental concept in calculus and other areas of mathematics. It allows for precise calculations and can be applied to a variety of functions and problems.

What is the difference between Riemann integrability and Riemann sum?

Riemann integrability is a concept that describes the ability to calculate the area under a curve using a specific method, while Riemann sum is a numerical technique used to approximate the area under a curve using a finite number of rectangles. Riemann integrability provides the exact value of an integral, while Riemann sum provides an estimate.

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