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Evaluate $$\int_0^1 \ln^2{\left(x^4+x^2+1\right)} \, \mathrm{d}x$$

First thing I saw is $x^4+x^2+1=(x^2+x+1)(x^2-x+1)$ so the integral is the same as:

\begin{gather*} \int_0^1\ln^2{\left(x^2+x+1\right)} \, \mathrm{d}x + \int_0^1\ln^2{\left(x^2-x+1\right)} \, \mathrm{d}x \\ + 2\int_0^1\ln{\left(x^2+x+1\right)}\ln{\left(x^2-x+1\right)} \, \mathrm{d}x \end{gather*}

I dont know if this helps though. The last integral above seems to be the hardest, but still I don't even know how to evaluate first 2 integrals (perhaps Feynman's method)?

Source: https://tieba.baidu.com/p/4794735082

Sangchul Lee
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  • @PeterForeman I saw it asked on a website and the other integrals have closed forms –  Aug 02 '20 at 18:32
  • Wolfram Alpha only gives a numeric value – saulspatz Aug 02 '20 at 18:33
  • @saulspatz Wolfram is weak with integrals like these –  Aug 02 '20 at 18:34
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    Perhaps after doing integration by parts you can partial fraction the resulting expression with that factoring, or you could use geometric series $$1+x^2+x^4 = \frac{x^6-1}{x^2-1}$$ – Ninad Munshi Aug 02 '20 at 18:41
  • @ThreeSidedCoin I added –  Aug 02 '20 at 19:01
  • By making the substitution $x \to x+1$, the sum of the first two integrals at the end can be rewritten as $$\int_{-1}^{1} \ln^2(x^2+x+1) dx$$ – Varun Vejalla Aug 02 '20 at 19:33
  • If all else fails, you might be able to do it by writing $x^4+x^2+1$ as a product of its complex roots, and then you won't have to worry about high powers of $x$. – Polygon Aug 02 '20 at 22:22
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    Using Henry Lee's substitution and simplifying a bit, I managed to rewrite the integral as $$\ln\left(\frac{4}{3}\right)\left(-2\ln\left(6\right)-\pi\sqrt{3}+8\right)+\frac{1}{\sqrt{2}}\int_{0}^{\sqrt{2}}\ln^{2}\left(1+\frac{\left(x^{2}+1\right)^{2}}{3}\right)dx$$ – Varun Vejalla Aug 03 '20 at 03:07
  • @VarunVejalla. Nice simplification but the last term still makes the problem : the antiderivative exists but its symbolic evaluation a t the bounds is killing. – Claude Leibovici Aug 03 '20 at 06:13
  • @ClaudeLeibovici To be honest, I don't know if what I did is considered a "simplification" - the integral seems to be just as intractable as the original. – Varun Vejalla Aug 03 '20 at 07:01
  • @VarunVejalla the integral you wrote could be done via Complex Analysis could you detail the simplifications you made – Zophikel Aug 15 '20 at 02:13
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    @Zophikel I didn't note down the exact steps, but I simplified $\frac{3}{4} \left( \tan^2(x)+1 \right) = \frac{3}{4} \sec^2(x)$, split the $\ln^2$ using this (which made $3$ integrals), and then solved the $3$ integrals (I think using the substitution $u = \sec(x)$). – Varun Vejalla Aug 17 '20 at 00:06

3 Answers3

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\begin{align}\boxed{I=\int_0^1 \ln^2(1+x^2+x^4)dx \\ = 32-12\ln 3-4\ln 2\ln 3-4 \pi \sqrt 3 +3\ln^2 3+2\pi \sqrt 3\ln 2+\frac{7\pi \ln 3}{\sqrt 3}\\ -8\sqrt 3 \operatorname{Ti}_2\left(\sqrt 3\right)+3\operatorname{Li}_2\left(-\frac13\right)+\operatorname{Li}_2\left(-3\right)-\operatorname{Li}_2\left(-8\right)}\end{align}

Where $\operatorname{Ti}_{2}(x)$ is the inverse tangent integral and $\operatorname{Li}_2(x)$ is the Dilogarithm.


To show this result we will start by using $(a+b)^2=2a^2+2b^2-(a-b)^2$ in order to rewrite the integral as: $$I=2\int_0^1 \ln^2(1+x+x^2)dx+2\int_0^1 \ln^2(1-x+x^2)dx -\int_0^1\ln^2\left(\frac{1+x+x^2}{1-x+x^2}\right)dx$$ $$\overset{x\to \frac{1-x}{1+x}}=\color{blue}{4\int_0^1 \frac{\ln^2\left(\frac{3+x^2}{(1+x)^2}\right)}{(1+x)^2}dx+4\int_0^1 \frac{\ln^2\left(\frac{1+3x^2}{(1+x)^2}\right)}{(1+x)^2}dx}\color{red}{-2\int_0^1 \frac{\ln^2\left(\frac{3+x^2}{1+3x^2}\right)}{(1+x)^2}dx}$$ $$\overset{x\to \frac{1}{x}}=\color{blue}{4\int_0^\infty \frac{\ln^2\left(\frac{3+x^2}{(1+x)^2}\right)}{(1+x)^2}dx}\color{red}{-\int_0^\infty \frac{\ln^2\left(\frac{3+x^2}{1+3x^2}\right)}{(1+x)^2}dx}$$ Now we will integrate by parts, in the same time we'll also simplify things using partial fractions. \begin{align}\Rightarrow I=3\ln^2 3+12\underbrace{\int_0^\infty \frac{\ln(3+x^2)}{3+x^2}dx}_{=I_1}+4\underbrace{\int_0^\infty \frac{\ln(1+3x^2)}{3+x^2}dx}_{=I_2}-32\underbrace{\int_0^\infty \frac{\ln(1+x)}{3+x^2}dx}_{=I_3(1)}\\ -16\underbrace{\int_0^\infty \frac{\ln(3+x^2)}{(1+x)^2}dx}_{=I_4}+32\underbrace{\int_0^\infty \frac{\ln(1+x)}{(1+x)^2}dx}_{=I_5} -24\underbrace{\int_0^\infty \frac{\ln(3+x^2)}{(3+x^2)(1+x)}dx}_{=I_6(1)}\\+8\underbrace{\int_0^\infty \frac{\ln(1+3x^2)}{(3+x^2)(1+x)}dx}_{=I_7(3)}+32\underbrace{\int_0^\infty \frac{\ln(1+x)}{(3+x^2)(1+x)}dx}_{=I_8(1)}\end{align} And all that's left to do is to evaluate each integral in order to obtain the closed form.


We obtain immediately, using $\int_0^\infty \frac{\ln(a+x^2)}{b+x^2}dx=\frac{\pi}{b}\ln(a+b)$ that: $$I_1=\int_0^\infty \frac{\ln(3+x^2)}{(3+x^2)}dx=\frac{\pi \ln 2}{\sqrt 3}+\frac{\pi \ln 3}{2\sqrt 3}$$ $$I_2=\int_0^\infty \frac{\ln(1+3x^2)}{(3+x^2)}dx=\frac{2\pi \ln 2}{\sqrt 3}$$


$$I_3(t)=\int_0^\infty \frac{\ln(1+tx)}{3+x^2}dx\Rightarrow I_3'(t)=\int_0^\infty\frac{x}{(1+tx)(3+x^2)} dx$$ $$=\frac{3\pi}{2\sqrt 3}\frac{t}{1+3t^2}-\frac{\ln 3}{2}\frac{1}{1+3t^2}-\frac{\ln t}{1+3t^2}$$ $$I_3(1)=\int_0^1 I_3'(t)dt=\frac{\pi \ln 2}{2\sqrt 3}-\frac{\pi \ln 3}{6\sqrt 3}+\frac{\operatorname{Ti}_2(\sqrt 3)}{\sqrt 3}$$ Where the inverse tangent integral appears after integrating by parts the last term as: $$\small \int_0^1 \frac{\ln t}{1+3t^2}dt\overset{IBP}=-\frac{1}{\sqrt 3}\int_0^1\frac{\arctan(\sqrt 3t)}{t}dt\overset{\sqrt 3 t=x}=-\frac{1}{\sqrt 3}\int_0^\sqrt 3\frac{\arctan x}{x}dx=-\frac{\operatorname{Ti}_2(\sqrt 3)}{\sqrt 3}$$


$$I_4=\int_0^\infty \frac{\ln(3+x^2)}{(1+x)^2}dx\overset{IBP}=\frac{3\pi}{4\sqrt 3}+\frac34\ln 3$$

$$I_5=\int_0^\infty\frac{\ln(1+x)}{(1+x)^2}dx\overset{IBP}=1$$


$$I_6(t)=\int_0^\infty \frac{\ln(3+tx^2)}{(3+x^2)(1+x)}dx\Rightarrow I_6'(t)=\int_0^\infty \frac{x^2}{(3+tx^2)(3+x^2)(1+x)}dx$$ $$=\frac{1}{8}\frac{\ln \left(\frac3t\right)}{3+t}+\frac{\pi}{8\sqrt 3}\frac{\sqrt t}{3+t}-\frac{1}{8}\frac{\ln t}{1- t}-\frac{\pi}{8\sqrt 3}\frac{1}{1+\sqrt t}$$ $$\small I_6(1)=\int_0^1I_6'(t)dt+\underbrace{\frac{\pi\ln 3}{8\sqrt 3}+\frac{\ln^2 3}{8}}_{=I_6(0)}=\frac{\ln 2 \ln 3}{4}-\frac18\operatorname{Li}_2\left(-\frac13\right)-\frac{\pi^2}{48}+\frac{\pi\ln 2}{4\sqrt 3}+\frac{\pi\ln 3}{8\sqrt 3}$$ Also that Dilogarithm comes from the first term, since: $$ \int_0^1 \frac{\ln\left(\frac{3}{t}\right)}{3+t}dt\overset{\frac{t}{3}=x}=-\int_0^\frac13 \frac{\ln t}{1+t}dt\overset{IBP}=2\ln 2\ln 3-\ln^2 3-\operatorname{Li}_2\left(-\frac13\right)$$


$$I_7(t)=\int_0^\infty \frac{\ln(1+tx^2)}{(3+x^2)(1+x)}dx\Rightarrow I_7'(t)=\int_0^\infty \frac{x^2}{(1+tx^2)(3+x^2)(1+x)}dx$$ $$=\frac{\pi}{8}\frac{\sqrt t}{1+t}+\frac{3\pi \sqrt t -\pi\sqrt 3}{8}\frac{1}{1-3t}-\frac18\frac{\ln t}{1+t}-\frac38\frac{\ln (3t)}{1-3t}$$ $$I_7(3)=\int_0^3I_7'(t)dt=\frac{\pi \ln 2}{2\sqrt 3}-\frac{\pi^2}{16}-\frac{\ln 2\ln 3}{4}-\frac{\operatorname{Li}_2(-3)}{8}-\frac18\operatorname{Li}_2(-8)$$ It's perhaps worth to mention here the last integral: $$\small \int_0^1 \frac{\ln(3t)}{1-3t}dt\overset{3t=x}=\frac13\int_0^9\frac{\ln x}{1-x}dx=\frac13\operatorname{Li}_2(1-x)\bigg|_0^9=\frac13\operatorname{Li}_2(-8)-\frac{\pi^2}{18}$$


$$I_8(t)=\int_0^\infty \frac{\ln(1+tx)}{(3+x^2)(1+x)}dx\Rightarrow I_8'(t)=\int_0^\infty\frac{x}{(1+tx)(3+x^2)(1+x)} dx$$ $$=\frac14\frac{\ln t}{1-t}+\frac{3\pi}{8\sqrt 3}\frac{1+t}{1+3x^2}+\frac{\ln 3}{8}\frac{3t-1}{1+3t^2}-\frac14\frac{\ln t}{1+3t^2}+\frac34\frac{t\ln t}{1+3t^2}$$ $$I_8(1)=\int_0^1 I_8'(t)dt=\frac{\pi\ln 2}{8\sqrt 3}+\frac{\ln 2\ln 3}{8}-\frac{\pi \ln 3}{24\sqrt 3}+\frac{\operatorname{Ti}_2(\sqrt 3)}{4\sqrt 3}+\frac{\operatorname{Li}_2(-3)}{16}$$

Zacky
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I would maybe try: $$x^4+x^2+1=(x^2+\frac12)^2+\frac34$$ and so: $$\int_0^1\ln^2(x^4+x^2+1)dx=\int_0^1\ln^2\left[(x^2+\frac12)^2+\frac{\sqrt{3}}{2}^2\right]dx$$ Now we know that: $$\tan^2u+1=\sec^2u$$ so by letting: $$\left[\frac{2}{\sqrt{3}}\left(x^2+\frac12\right)\right]=\tan(u)$$ $$\frac{4}{\sqrt{3}}xdx=\sec^2udu$$ we get: $$I=\frac{\sqrt{3}}4\int\limits_{\frac\pi6}^{\frac\pi3}\ln^2\left[\frac32\tan^2u+\frac32\right]\left(\frac{\sqrt{3}\tan(u)-1}{2}\right)^{-1/2}\sec^2(u)du$$ and whilst the inside of the natural log simplifies nicely the rest is still rather ugly so I'm unsure if I can get a nice result from it but its worth a try :)

Henry Lee
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    Numerically, the two integrals don't seem to be equal. It should equal $\approx 0.247$, but your integral is $\approx 1.238$. – Varun Vejalla Aug 02 '20 at 21:33
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    I think it should be $$\ln^2\left[\frac{3}{\color{red}{4}}\tan^2u+\frac{3}{\color{red}{4}}\right]$$ in the integrand. – Varun Vejalla Aug 02 '20 at 21:44
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If you enjoy very, bery long formulae, try another CAS to see the antiderivative (it would take pages to type).

The problem is that the symbolic evaluation at the bounds almost killed my computer. So, numerical evaluation of the symbolic results.

At the upper bound $$23.67702048724287969803516653795923942977580171907339873883701501010155$$ At the lower bound $$23.43008216952775709246803606740565726521761645296916931243549426722402$$

Then, for the definite integral $$0.2469383177151226055671304705535821645581852661042294264015207428775327$$ which is not recognized by inverse symbolic calculators.

I seriously wonder if the square applies to the logarithm and not to its arguments; if this was the case, the problem would be quite simple.