Does this integral converge or diverge?

  • Thread starter Dell
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In summary, the given integral \int\frac{dx}{ln(x)} (from 0 to 1) is divergent. This can be seen by transforming the integral using the substitution t=ln|x| and evaluating the limits at negative infinity and zero, which show that the integral of exp(t)/t converges at negative infinity and diverges at zero. Other methods were also attempted, such as finding a similar function with known convergence or divergence properties, but ultimately the conclusion remained the same. The area between 0.4 and 0.6 was not taken into account since the function is constant there.
  • #1
Dell
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the given integral is:

[tex]\int[/tex][tex]\frac{dx}{ln(x)}[/tex] (from 0 to 1)


now i have a problem at both ends of the integral, because i have ln(0) is undefinde and ln(1) is 0 and 1/0 is undefined, so i can integrate from 0 to 0.5 and 0.5 to 1, but i tried to integrate this and just couldnt, i tried substitution
t=ln(x)
x=et
dt=dx/x

and then tried integration in parts but this just didnt work,

so then i thought maybe i need to find a similar function which i know more about or can easily find more about, but again i have problems at both limits, so i think i need 2 separate functions, since i know that 1/ln(x) is negative in the limits 0-1, i will take the abs value of 1/ln(x) and look for positive functions...
i am looking for an integral that is either
*bigger than my original integral and converges
*smaller than my original integral and diverges

what i found was
[tex]\int[/tex]dx/[tex]\sqrt{x}[/tex] (from 0 to 0.4) which converges and is bigger than [tex]\int[/tex]dx/|ln(x)| (from 0 to 0.4 )

and my second integral is [tex]\int[/tex]1/x (from 0.6 to 1) which diverges and is smaller than [tex]\int[/tex]dx/|ln(x)| (from 0.6 to 1 )

therefore i know that the integral [tex]\int[/tex][tex]\frac{dx}{ln(x)}[/tex] (from 0 to 1) diverges near 1, and converges near 0, so all in all it diverges.

is this okay? is it alright that i didnt take into account the area between 0.4 and 0.6 since the function is constant there anyway??

can you see any better way to solve this??
 
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  • #2
Dell said:
is this okay? is it alright that i didnt take into account the area between 0.4 and 0.6 since the function is constant there anyway??

can you see any better way to solve this??

I came to a similar conclusion using your first substitution t=ln|x|. I found this easier to visualize. The lower limit of zero transforms to negative infinity and it is clear that the integral of exp(t)/t converges at negative infinity. However the upper limit of 1 transforms to zero and it is clear that the integral of exp(t)/t diverges at zero, since the numerator goes to unity and the denominator is a divergent 1/t form.
 

1. What is the definition of a convergent integral?

A convergent integral is a type of mathematical calculation in which the limit of the function being integrated approaches a finite value as the limits of integration become infinitely large or small. In other words, the integral approaches a well-defined number as the boundaries of the function become larger or smaller.

2. How do you determine if an integral is convergent or divergent?

To determine if an integral is convergent or divergent, you can use a variety of tests such as the comparison test, limit comparison test, and the integral test. These tests involve comparing the given integral to a known integral that is either convergent or divergent.

3. What is the relationship between convergence and divergence in integrals?

Convergence and divergence are two opposite concepts in integrals. A convergent integral approaches a finite value as the boundaries of the function become infinitely large or small, while a divergent integral does not approach a well-defined number and instead goes to infinity or negative infinity.

4. Are there any common functions that always result in a convergent integral?

Yes, there are several common functions that always result in a convergent integral. These include polynomial functions, exponential functions, and trigonometric functions. However, the specific boundaries and coefficients of these functions can affect whether the integral is convergent or divergent.

5. Can an integral be both convergent and divergent?

No, an integral cannot be both convergent and divergent at the same time. It can only be one or the other. If an integral meets the criteria for both convergence and divergence, it is said to be indeterminate and requires further analysis using more advanced techniques.

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