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### Integrability

Posted: **Thu Jan 22, 2015 4:10 pm**

by **Rabeeb**

**(1)**A function is not differentiable at a point if it is not continuous at that point or it is a corner point(that is L.H.S derivative and R.H.S derivative differ in value).

**What are all the cases in which a function doesn't have (a)an indefinite integral or (b)a definite integral?**

**(2) How do we evaluate the integral $\int x^{-1} dx$** (talking about indefinite integral, a general formula)?

### Re: Integrability

Posted: **Sun Feb 01, 2015 7:22 pm**

by **photon**

**1.**If a function doesn't "have" indefinite integral , then it doesn't "have" definite integral too because they are both same thing except the constant gets cancelled in definite integral and we get a definite value for 2 given limits . Now some functions (like $\ln(\ln x),x^x,\sin(\sin x)$ ) can not be integrated . These are called non-elementary functions. As far as I know , although they can't be integrated , their integral value between 2 limits are

*calculated approximately* . I dont know how though(haven't studied yet!

).

Integral's geometric outcome is calculating area covered by $f(x)$ with $X$ axis . Even we can't calculate non-elementary function's integral directly , that doesn't mean the function doesn't cover any area . We calculate this

*approximately* .

**2.** While starting derivative chapters you may learn that $\displaystyle \frac{d}{dx}(\ln x)=\frac{1}{x}$ ,

as Integration is the reverse process of differentiation [I think here you will get a nice

explanation about reverse relation] ,

$\displaystyle \int \frac{1}{x}dx=\ln x+c$ .

### Re: Integrability

Posted: **Tue Feb 03, 2015 12:01 am**

by **Rabeeb**

Well.. But can we integrate all discontinuous functions as well??

### Re: Integrability

Posted: **Fri Feb 06, 2015 9:41 am**

by **Phlembac Adib Hasan**

Well, some non-elementary functions can be integrated (Yeah, the blessings of power series.

But this is not a general approach, as they don't always converge to the original function.) See more here:

http://en.wikipedia.org/wiki/Nonelementary_integral
@Rabeeb, here's the answer of your question:

http://math.stackexchange.com/questions ... -functions
http://math.stackexchange.com/questions ... integrable