Improper integral

  • Thread starter aceetobee
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  • #1
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Can someone explain why the following improper integral diverges?

Integral 1/x dx from -1 to 1

I know if you break it up the individual integrals (from -1 to 0 and 0 to 1) diverge to negative infinity and infinity, whose sum is indeterminant in general, but the symmetry of the integral suggests it "should be zero".

Thanks!
 

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  • #2
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OK... well, a little research and I think I answered my own question.

It really is an ill-definied integral, because when broken up into the limit of two separate integrals, there are an infinite number of ways this can be done, with one side approaching zero at a different speed than the other.

I guess the Cauchy Principle Value would be zero, but there are other possiblilities, so it diverges.

Please correct me if I'm wrong on this...
 
  • #3
HallsofIvy
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If [itex]\int_b^c f(x)dx[/itex] is "improper" because f(a) is not defined, with b< a< c, then the integral is DEFINED as:
[tex]lim_{x_1->a^-}\int_b^{x_1}f(x)dx+ lim_{x_2->a^+}\int_{x_2}^cf(x)dx[/tex].

Since the anti-derivative of 1/x is ln|x|, neither of those limits exists when a= 0.

The Cauchy Principal Value, on the other hand is:
[tex]lim_{x_1->a}\left(\int_b^{x_1}f(x)dx+ \int_{x_1}^cf(x)dx\right)[/tex].
Since the limit is taken after both integrals are done, we can cancel the "ln|x1|" terms before the limit and just have ln|c|-ln|b|.
 
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  • #4
Galileo
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aceetobee said:
It really is an ill-definied integral, because when broken up into the limit of two separate integrals, there are an infinite number of ways this can be done, with one side approaching zero at a different speed than the other.

I guess the Cauchy Principle Value would be zero, but there are other possiblilities, so it diverges.

Please correct me if I'm wrong on this...
That's totally correct.
 

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