Show this silly function is Riemann-integrable and find the integral

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Homework Help Overview

The discussion revolves around the Riemann integrability of a specific function defined on the interval [0,1]. The function takes the value 0 for irrational inputs and 1/q for rational inputs expressed in lowest terms as p/q. Participants are exploring the conditions under which this function can be shown to be integrable.

Discussion Character

  • Exploratory, Assumption checking, Conceptual clarification

Approaches and Questions Raised

  • The original poster attempts to demonstrate integrability by analyzing the set of discontinuities, questioning whether this set is indeed the rationals in the interval. They express uncertainty regarding the implications of the p/q representation and continuity around irrational numbers.
  • Another participant questions the density of rational numbers within an interval, prompting further exploration of the function's behavior.
  • A subsequent post introduces a method to define a finite set of rationals based on a chosen integer, suggesting a potential approach to establishing continuity conditions.

Discussion Status

The discussion is active, with participants engaging in exploratory reasoning and questioning assumptions about the function's properties. Some guidance has been offered regarding the construction of sets of rational numbers, but no consensus or resolution has been reached yet.

Contextual Notes

Participants are navigating the complexities of the function's definition and its implications for integrability, particularly focusing on the measure of discontinuities and the behavior of rational numbers in relation to irrationals.

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Homework Statement


See the title. The silly function in question is f:[0,1]-->R with f(x)=0 if x is irrational, and f(x)=1/q if x is rational and of the form x=p/q where p and q have no common factor.

Homework Equations


The Attempt at a Solution



I'm like 100% sure that I must show it is integrable by showing that the set of discontinuities is of measure zero and the natural assumption is that this set of discontinuities is the rationals in [0,1], but how do I show that?

I feel there is something I am missing about the p/q representation thing. If f is continuous on the irrational, then it must be that given an e>0 and an irrational y, there is a little disk around it y such that all rationals in that disk have 1/q<e. How come?
 
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How many rationals are there in an interval?
 
Ok, if y is irrational and you are given e>0, let Q be an integer such that Q>1/e. Now let S be the set of rationals {p/q} where q<=Q. S is a finite set. So let the radius of the disk be delta=inf(|y-S|)>0.
 
That is ingenious!
 

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