# $1+x^n$ in Fractions?

Could you calculate

$\large \int \frac{1}{1+x^n} dx$

for every positive integer $n$? Note by Pepper Mint
3 years, 8 months ago

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Ok......What if someone replaces the + sign with a - sign........Can we solve it and generalize it??

- 3 years, 4 months ago

That is a good point we can use its series expansion... WAIT THE SERIES EXPANSION!!! We can use it to maybe SOLVE THE INTEGRAL (in series form BUT WHO CARES)!!! $\frac { 1 }{ 1+x } =1-x+{ x }^{ 2 }-{ x }^{ 3 }+{ x }^{ 4 }-...=\sum _{ n=0 }^{ \infty }{ { (-1) }^{ n }{ x }^{ n } }$ $\frac { 1 }{ 1+{ x }^{ 2 } } =1-{ x }^{ 2 }+{ x }^{ 4 }-{ x }^{ 6 }+{ x }^{ 8 }-...=\sum _{ n=0 }^{ \infty }{ { (-1) }^{ n }{ x }^{ 2n } }$ $\frac { 1 }{ 1+{ x }^{ 3 } } =1-{ x }^{ 3 }+{ x }^{ 6 }-{ x }^{ 9 }+{ x }^{ 12 }-...=\sum _{ n=0 }^{ \infty }{ { (-1) }^{ n }{ x }^{ 3n } }$ So: $\frac { 1 }{ 1+{ x }^{ k } } =1-{ x }^{ k }+{ x }^{ 2k }-{ x }^{ 3k }+{ x }^{ 4k }-...=\sum _{ n=0 }^{ \infty }{ { (-1) }^{ n }{ x }^{ kn } }$ and...: $\int { \frac { 1 }{ 1+{ x }^{ k } } dx } =\int { (1-{ x }^{ k }+{ x }^{ 2k }-{ x }^{ 3k }+{ x }^{ 4k }-...)dx } =\int { \sum _{ n=0 }^{ \infty }{ { (-1) }^{ n }{ x }^{ kn } } dx }$

Simplifying the integral of the right: (Not including the +C in the integral) $\int { \sum _{ n=0 }^{ \infty }{ { (-1) }^{ n }{ x }^{ kn } } dx } =\sum _{ n=0 }^{ \infty }{ \int { { (-1) }^{ n }{ x }^{ kn }dx } } =\sum _{ n=0 }^{ \infty }{ \frac { { (-1) }^{ n }{ x }^{ kn+1 } }{ kn+1 } } =x-\frac { { x }^{ k+1 } }{ k+1 } +\frac { { x }^{ 2k+1 } }{ 2k+1 } -\frac { { x }^{ 3k+1 } }{ 3k+1 } +\frac { { x }^{ 4k+1 } }{ 4k+1 } -...$ ... which is so far all the work that can be done, without using any special functions.

Thus: $\int { \frac { 1 }{ 1+{ x }^{ k } } dx } =\sum _{ n=0 }^{ \infty }{ { \frac { { (-1) }^{ n }{ x }^{ kn+1 } }{ kn+1 } } }$ Done

- 3 years, 4 months ago

Just glancing at it and some solutions computed with WolframAlpha, it looks like you have to use partial fractions to decompose it and then integrate term-by-term, which makes me unsure about whether or not a closed-form solution exists...

- 3 years, 4 months ago

Integrate term by term... that is correct. In fact, the terms you need to integrate are actually very surprising.. x^n and -x^k! The answer is that $\int { \frac { 1 }{ 1+{ x }^{ k } } dx } =\sum _{ n=0 }^{ \infty }{ { \frac { { (-1) }^{ n }{ x }^{ kn+1 } }{ kn+1 } } }$!

- 3 years, 4 months ago

You can see the power rule in the summation!

- 3 years, 4 months ago

Anyway, we know that: $\int { \frac { 1 }{ 1+{ x } } } dx=\ln { (1+x) } +C$ and $\int { \frac { 1 }{ 1+{ x }^{ 2 } } } dx=\arctan { x } +C$ but $\int { \frac { 1 }{ 1+{ x }^{ 3 } } } dx$ is a mess...

(Just in case you don't believe me): $\int { \frac { 1 }{ 1+{ x }^{ 3 } } } dx=-\frac { \ln { |{ x }^{ 2 }-x+1| } -2(\ln { |x+1| } +\sqrt { 3 } \arctan { (\frac { 2x-1 }{ \sqrt { 3 } } ) } ) }{ 6 } +C$

- 3 years, 4 months ago

@Pepper Mint Well, we can solve this sort of definite integral ranging from 0 to infinity.........this is simply using Beta function....!!!

- 3 years, 1 month ago

But it won't help solve for the indefinite integral. hmm

- 3 years, 1 month ago

@Pepper Mint Have a look at this paper...

- 2 years, 4 months ago