Convergence of integral for $0 \le a <1$

In summary, by using the given formula (1) and deriving the function (2), it can be seen that the integral converges for $0 \le a <1$ as the function approaches $-x^{a-2}$ for large x and $\frac{-x^{a}}{6}$ for small x. Therefore, combining these two conditions, the integral converges for $-1 < a < 1$.
  • #1
polygamma
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0
How do you show that $\displaystyle \int_{0}^{\infty} \Big( \frac{x^{a-1}}{\sinh x} - x^{a-2} \Big) \ dx $ converges for $0 \le a <1$?
 
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  • #2
Random Variable said:
How do you show that $\displaystyle \int_{0}^{\infty} \Big( \frac{x^{a-1}}{\sinh x} - x^{a-2} \Big) \ dx $ converges for $0 \le a <1$?

Remembering that ...

$\displaystyle \frac{1}{\sinh x} = \frac{1}{x} - \frac{x}{6} + \frac{7}{360}\ x^{2} - ...$ (1)

... You derive that is...

$\displaystyle \frac{x^{a-1}}{\sinh x} - x^{a-2} = x^{a-2}\ (1 - \frac{x^{2}}{6} + \frac{7}{360}\ x^{4} + ... -1) = - x^{a}\ (\frac{1}{6} - \frac{7}{360}\ x^{2} + ...)$ (2)

... so that the function around x=0 is $\displaystyle \sim - \frac{x^{a}}{6}$. For 'large x' the function is $\displaystyle \sim - x^{a-2}$ so that, combining the two conditions, the integral should converge for $-1 < a < 1$...

Kind regards

$\chi$ $\sigma$
 

FAQ: Convergence of integral for $0 \le a <1$

What is the definition of convergence of an integral?

The convergence of an integral refers to the behavior of the integral as the limits of integration approach a certain value. If the integral has a finite value as the limits approach this value, it is said to be convergent. If the integral has an infinite value, it is said to be divergent.

How can I determine if an integral is convergent or divergent?

One method is to use the Comparison Test, where the integral is compared to a known convergent or divergent integral. Another method is the Limit Comparison Test, where the ratio of the two integrals is taken and its limit is evaluated. If the limit is a finite, non-zero value, then both integrals will have the same convergence behavior.

What are some common types of integrals that are known to be convergent or divergent?

Integrals involving power functions, exponential functions, logarithmic functions, and trigonometric functions are commonly known to have convergence or divergence behavior. For example, the integral of 1/x from 1 to infinity is divergent, while the integral of e^(-x) from 0 to infinity is convergent.

Can an integral be both convergent and divergent?

No, an integral can only have one of these two behaviors. However, it is possible for an integral to have both a convergent and divergent part. This is known as an improper integral, where the limits of integration involve infinity or a point of discontinuity.

What is the importance of determining the convergence of an integral?

Convergence of an integral is important in many areas of mathematics and science, such as in calculus, physics, and statistics. It allows us to determine the behavior of a function and make predictions about its behavior in the future. It also helps us to evaluate the accuracy of numerical methods used to approximate integrals.

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