Improper Integrals: Definite & Indefinite | Bounds -1 to 1

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Discussion Overview

The discussion revolves around the concept of improper integrals, specifically focusing on the definite integral of the function 1/x over the bounds -1 to 1, and the nature of indefinite integrals, particularly for discontinuous functions. Participants explore the definitions and properties of integrals, addressing both theoretical and conceptual aspects.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants note that the definite integral of 1/x from -1 to 1 is improper due to the discontinuity at x=0.
  • There is a discussion about the indefinite integral of 1/x, which is expressed as ln|x|, with some participants questioning the validity of finding indefinite integrals for discontinuous functions.
  • One participant emphasizes that an indefinite integral is defined at a point rather than over an interval, suggesting that the issue primarily arises at x=0.
  • Another participant points out that the function ln|x| is continuous on two disjoint intervals, which leads to the conclusion that the antiderivative can be defined piecewise for x < 0 and x > 0.
  • Some argue that continuity is not a requirement for integrability, asserting that antiderivatives can exist for functions with discontinuities, although expressing them as elementary functions may be complex.
  • A participant expresses a critical view of Riemann integrals, describing them in a negative light.

Areas of Agreement / Disagreement

Participants express differing views on the nature of indefinite integrals for discontinuous functions, with some asserting that antiderivatives can exist despite discontinuities, while others focus on the implications of continuity and the definition of integrals. The discussion remains unresolved regarding the broader implications of these points.

Contextual Notes

Participants reference the piecewise definition of antiderivatives and the potential complexities in expressing them, indicating that there may be limitations in the definitions and assumptions surrounding integrability and continuity.

FS98
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if I wanted to take the definite integral of 1/x with respect to x, with the bounds -1 and 1, the integral would be improper.

What about the indefinite integral? We can find the indefinite integral of 1/x to be ln|x|. Can we find the indefinite integral of discontinuous functions?
 
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FS98 said:
if I wanted to take the definite integral of 1/x with respect to x, with the bounds -1 and 1, the integral would be improper.

What about the indefinite integral? We can find the indefinite integral of 1/x to be ln|x|. Can we find the indefinite integral of discontinuous functions?
The function f(x) = ln|x| is defined and continuous on two disjoint intervals: ##(-\infty, 0)## and ##(0, \infty)##. An indefinite integral is an antiderivative, a function, while a definite integral represents a number. Since ln|x| is differentiable on either of the two intervals listed above, and its derivative is 1/x, then it's valid to say that ##\int \frac {dx} x = \ln|x|## plus a constant.
 
It is important to note that an indefinite integral is not over an interval, but only at a point, so your question raises a problem only at x=0.
 
Mark44 said:
The function f(x) = ln|x| is defined and continuous on two disjoint intervals: ##(-\infty, 0)## and ##(0, \infty)##. An indefinite integral is an antiderivative, a function, while a definite integral represents a number. Since ln|x| is differentiable on either of the two intervals listed above, and its derivative is 1/x, then it's valid to say that ##\int \frac {dx} x = \ln|x|## plus a constant.
I am nitpicking here, but it seems to me that we had a discussion around this sort of topic some time ago. The antiderivative would be a family of functions which are definable piecewise as ##\ln|x|## plus a constant for x < 0 and ##\ln|x|## plus a [possibly different] constant for x > 0.
 
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FS98 said:
if I wanted to take the definite integral of 1/x with respect to x, with the bounds -1 and 1, the integral would be improper.

What about the indefinite integral? We can find the indefinite integral of 1/x to be ln|x|. Can we find the indefinite integral of discontinuous functions?
Continuity is not necessary for integrability. So, yes, we certainly can find antiderivatives for functions containing discontinuities. Whether you can express these antiderivatives as compositions of elementary functions is another matter, however.

Riemann integrals are the work of the devil, I tell you.
 

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