# difficult integration: int [ 1 / (1 + x^4) ] dx

Show 40 post(s) from this thread on one page
Page 1 of 2 12 Last
• 03-25-2008, 12:23 AM
dts5044
difficult integration: int [ 1 / (1 + x^4) ] dx
integrate:

dx/ (1 + x[sup:2fussihg]4[/sup:2fussihg])

I've tried by parts using u = 1 / (1 + x[sup:2fussihg]4[/sup:2fussihg]) and dv = dx, but that only increased the power of the function

I also tried using the substitution x = SQRT(tant), with dx = sec[sup:2fussihg]2[/sup:2fussihg](t) / (2SQRT(tant)). This seemed to simplify it a bit but I still got stuck integrating.
• 03-25-2008, 09:15 AM
tkhunny
Re: difficult integration
Have you considered factoring?

$$x^{4}+1 = (x^{2}+\sqrt{2}x+1)(x^{2}-\sqrt{2}x+1)$$

That may lead somewhere if you are courageous.
• 03-25-2008, 10:00 AM
stapel
Quote:

Originally Posted by dts5044
integrate: dx/ (1 + x[sup:wq5pahuc]4[/sup:wq5pahuc])

The Integrator would appear to suggest that factorization, as nasty as that will be, is the way to go. :shock:

Eliz.
• 03-25-2008, 12:31 PM
tkhunny
Re: difficult integration: int [ 1 / (1 + x^4) ] dx
...and the associated partial fractions are no additional picnic.

This is an excellent argument for computer-based examination of such things. Is all this pain really necessary?
• 03-25-2008, 02:45 PM
Dr. Flim-Flam
Re: difficult integration: int [ 1 / (1 + x^4) ] dx
1/(1+x^4) = 1/[1-(-x^4)} = Sum [(-1)^n*x^(4n)], n goes from 0 to infinity.

integral of [1/(1+x^4)]dx = Sum[(-1)^n*x^(4n+1)]/(4n+1), n goes from 0 to infinity.

I'll leave as an exercise to find the interval of convergence.
• 03-25-2008, 07:15 PM
soroban
Re: difficult integration: int [ 1 / (1 + x^4) ] dx
Hello, dts5044!

Want to see a truly far-out solution?

Quote:

$$\int\frac{dx}{1 + x^4}$$

$$\text{We have: }\;\frac{1}{x^4 + 1} \;=\;\frac{1}{2}\bigg[\frac{x^2 + 1 - x^2 + 1}{x^4+1}\bigg] \;=\;\frac{1}{2}\bigg[\frac{x^2+1}{x^4+1} - \frac{x^2-1}{x^4+1}\bigg]$$

$$\text{Divide top and bottom by }x^2\!:\;\;\frac{1}{2}\left[\frac{\frac{x^2+1}{x^2}}{\frac{x^4+1}{x^2}} - \frac{\frac{x^2-1}{x^2}}{\frac{x^4+1}{x^2}} \right]$$

$$\text{And we will integrate: }\;\frac{1}{2}\left[\int\frac{\left(1 + \frac{1}{x^2}\right)\,dx}{x^2 + \frac{1}{x^2}} - \int\frac{\left(1-\frac{1}{x^2}\right)\,dx}{x^2 + \frac{1}{x^2}}\right]$$ .[1]

You think that's weird? . . . Just wait!

$$\text{Let }\,u \:=\:x - \frac{1}{x}\quad\Rightarrow\quad du \:=\:\left(1 + \frac{1}{x^2}\right)\,dx$$

$$\text{Let }\,v \;=\:x + \frac{1}{x}\quad\rightarrow\quad dv \:=\:\left(1 - \frac{1}{x^2}\right)\,dx$$

. . $$u^2\:=\:x^2 - 2 + \frac{1}{x^2}\quad\Rightarrow\quad u^2+2 \:=\:x^2 + \frac{1}{x^2}$$

. . $$v^2 \:=\:x^2 + 2 + \frac{1}{x^2}\quad\Rightarrow\quad v^2-2 \:=\:x^2 + \frac{1}{x^2}$$

Substitute into [1]:

. . . . $$\frac{1}{2}\left[\int\frac{du}{u^2 + 2} - \int\frac{dv}{v^2-2}\right]$$

Just integrate and back-substitute . . .

~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~

I wish I could take credit for this innovative approach,
. . but it was posted in February 2007 by a tutor named "commutative".
(And I can't locate the site where it appeared.)

• 03-25-2008, 07:43 PM
Subhotosh Khan
Re: difficult integration: int [ 1 / (1 + x^4) ] dx
If complex number is allowable then:

$$x^4 + 1 = (x^2)^2 - (i)^2 = (x^2 - i)\cdot (x^2 + i)$$

Break into partial fractions and then standard integration.
• 03-25-2008, 09:13 PM
pka
Re: difficult integration: int [ 1 / (1 + x^4) ] dx
I have not posted on this topic because it is a perfect example of what I think is wrong the mathematics education today.
The question does not ask if the integral exists.
That is a theoretical question that is important if we ought to continue.
If the function is integrable then any reasonable CAS will find the value of integrals.
How one finds the so-called ‘primitive’ (antiderivative) is really moot!
So my question is: “why are you concerned?”
• 03-26-2008, 08:34 AM
tkhunny
Re: difficult integration: int [ 1 / (1 + x^4) ] dx
I wondered how long it would take. I was baiting you. :)
• 03-26-2008, 08:50 AM
galactus
Re: difficult integration: int [ 1 / (1 + x^4) ] dx
'Commutative' is a very learned contributor on SOS Math
Show 40 post(s) from this thread on one page
Page 1 of 2 12 Last