Example of a non-integrable function f , such that |f| and f^2 are integrable?

  • Context: Graduate 
  • Thread starter Thread starter irresistible
  • Start date Start date
  • Tags Tags
    Example Function
Click For Summary

Discussion Overview

The discussion revolves around identifying a function \( f: [a,b] \to \mathbb{R} \) that is non-integrable while both \( |f| \) and \( f^2 \) are integrable on the same interval. The scope includes analysis concepts related to integrability and discontinuous functions.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant suggests that \( f \) must be discontinuous for \( |f| \) and \( f^2 \) to be integrable while \( f \) itself is not.
  • Another participant proposes a specific function: \( f(x) = 1 \) for rational \( x \) and \( f(x) = -1 \) for irrational \( x \).
  • A different viewpoint mentions that the integrability of \( f \) depends on whether one is using Newton or Riemann integrals, implying that the context of integration matters.
  • One participant expresses gratitude for the suggestions, indicating a collaborative atmosphere.

Areas of Agreement / Disagreement

Participants appear to agree on the need for \( f \) to be discontinuous, but there is no consensus on the specific function or the implications of different types of integrals.

Contextual Notes

There are unresolved assumptions regarding the definitions of integrability and the types of integrals being referenced, which may affect the discussion.

irresistible
Messages
15
Reaction score
0
I'm looking for a function
f:[a,b] -> R such that |f| and f2 are integrable on [a,b]
any helps?
 
Physics news on Phys.org
To start with, f must be discontinuous (a continuous function on a closed interval is integrable). Can you think of a function f such that f is discontinuous, but |f| (and thus f2 = |f|2) is continuous?This is really an analysis question, so it really belongs in that forum. :)[/size]
 
Last edited:
f(x)= 1 for x rational,
-1 for x irational
 
Don't just give him an answer. ;)

(That's the exact same function I was thinking of, though.)
 
I forgot to say: Only if you are dealing with Newton or rieman integrals, otherwise f(x) is integrable.
 
Oh I don't know why I didn't think of that!
Thank you so much to both of youuuu!
:smile:
 

Similar threads

  • · Replies 6 ·
Replies
6
Views
4K
  • · Replies 20 ·
Replies
20
Views
5K
  • · Replies 4 ·
Replies
4
Views
2K
  • · Replies 1 ·
Replies
1
Views
3K
  • · Replies 4 ·
Replies
4
Views
2K
  • · Replies 20 ·
Replies
20
Views
3K
  • · Replies 8 ·
Replies
8
Views
2K
  • · Replies 11 ·
Replies
11
Views
2K
  • · Replies 3 ·
Replies
3
Views
3K
  • · Replies 10 ·
Replies
10
Views
6K