Vrftsjtryk

Here are some links to a few integrals: 1
(Big list, but not all of them got the right answer).
From AoPS: 2, 3 , 4. Some that are solvable with Feynman's trick: here.

As for my favourites (most of them appeared on Romanian Mathematical Magazine), some are: $$I_1=int_0^frac{pi}{2} frac{arctan(tan xsec x)}{tan x +sec x}dx=frac{pi}{2}ln 2 -frac{pi}{6}ln(2+sqrt 3)$$
$$I_2=int_0^infty expleft(-frac{3x^2+15}{2x^2+18}right)cosleft(frac{2x}{x^2+9}right)frac{dx}{x^2+1}=frac{pi}{e}$$
$$I_3=int_0^1 frac{ln^2 (1+x) (ln^2 (1+x) +6ln^2(1-x))}{x}dx=frac{21}{4}zeta(5)$$
$$I_4=int_0^infty frac{ln x}{(pi^2+ln^2 x)(1+x)^2}frac{dx}{sqrt x}=-frac{pi}{24}$$
$$I_5=int_0^infty frac{1-cos x}{8-4xsin x +x^2(1-cos x)}dx=frac{pi}{4}$$
$$I_6=int_0^infty frac{arctan x}{x^4+x^2+1}dx=frac{pi^2}{8sqrt{3}}-frac{2}{3}G+frac{pi}{12}ln(2+sqrt{3})$$
$$I_7=int_0^infty frac{ln(1+x)}{x^4-x^2+1}dx=frac{pi}{6}ln(2+sqrt 3)+frac23 G -frac{pi^2}{12 sqrt 3}$$
$$I_8=int_0^infty left(sqrt{x}-sqrt{sqrt{1+x^2}-1}right)sin{x}dx=frac12 sqrt{frac{pi}{2}}frac{e-1}{e}$$
$$I_{9}=int_0^{frac{pi}{4}} lnleft(2+sqrt{1-tan^2 x}right)dx = frac{pi}{2}lnleft(1+sqrt{2}right)+frac{7pi}{24}ln2-frac{pi}{3}lnleft(1+sqrt{3}right)-frac{G}{6}$$
$$I_{10}=int_{-infty}^infty frac{sin left(x-frac{1}{x}right) }{x+frac{1}{x}}dx=frac{pi}{e^2}$$
$$I_{11}=int_{-infty}^infty frac{cos left(x-frac{1}{x}right) }{left(x+frac{1}{x}right)^2}dx=frac{pi}{2e^2}$$
$$I_{12}=int_0^1 frac{operatorname{arcsinh}sqrt{frac{1}{x+2}}}{sqrt{3-2x-x^2}}dx=frac23 G$$
$$I_{13}=int_0^frac{pi}{2} x^2 sqrt{tan x}dx=frac{sqrt{2}pi(5pi^2+12piln 2 - 12ln^22)}{96}$$
$G$ stands for Catalan's constant. $I_8$ and $I_{12}$ are one of Cornel Ioan Valean's integral, I heard that his book is coming out soon which surely contains some exotic ones. I can't prove them here, but if you desire to evaluate one of those integrals and get stuck feel free to ask for help/hints.

Here are some of my favorites:
$$int_0^pi sin^2Big(x-sqrt{pi^2-x^2}Big)dx=frac{pi}{2}$$
$$int_0^infty frac{ln(x)}{(1+x^{sqrt 2})^sqrt{2}}dx=0$$
$$int_0^infty frac{dx}{(1+x^{1+sqrt{2}})^{1+sqrt{2}}}=frac{1}{sqrt{2}}$$
$$int_{-infty}^infty ln(2-2cos(x^2))dx=-sqrt{2pi}zeta(3/2)$$
$$int_0^infty frac{text{erf}^2(x)}{x^2}dx=frac{4ln(1+sqrt{2})}{sqrt{pi}}$$
$$int_0^infty frac{x^{3}ln(e^x+frac{x^3}{6}+frac{x^2}{2}+x+1)-x^4}{frac{x^3}{6}+frac{x^2}{2}+x+1}=frac{pi^2}{2}$$
$$int_0^{pi/2} ln(x^2+ln^2(cos(x)))dx=piln(ln(2))$$
$$int_0^infty frac{arctan(2x)+arctan(x/2)}{x^2+1}dx=frac{pi^2}{4}$$
$$int_0^{pi/2}frac{sin(x+100tan(x))}{sin(x)}dx=frac{pi}{2}$$
$$int_0^1 frac{xln(1+x+x^4+x^5)}{1+x^2}dx=frac{ln^2(2)}{2}$$
$$int_0^{1/2}sin(8x^4+x)cos(8x^4-x)cos(4x^2)xdx=frac{sin^2(1)}{16}$$

$$int_0^{2pi} sqrt{2+cos(x)+sqrt{5+4cos(x)}}dx=4pi$$

And here are four extremely exotic scrumptious integrals:

$$int_0^1 frac{sin(pi x)}{x^x (1-x)^{1-x}}dx=frac{pi}{e}$$
$$int_{-infty}^infty frac{dx}{(e^x-x)^2+pi^2}=frac{1}{1+Omega}$$

$$int_0^infty frac{3pi^2+4(z-sinh(z))^2}{[3pi^2+4(z-sinh(z))^2]^2+16pi^2(z-sinh(z))^2}dz=frac{1}{8+8sqrt{1-w^2}}$$

$$int_0^{pi/2}ln|sin(mx)|ln|sin(nx)|dx=frac{pi^3}{24}frac{gcd^2(m,n)}{mn}+frac{pi ln^2(2)}{2}$$

...where $Omega$ is the Omega Constant, $w$ is the Dottie Number, and $m,ninmathbb N$.

I'm partial to the one in this question
What is the Centroid of $z=frac{1}{(1-itau)^{i+1}}, tauin (-infty,infty)$ .

I found a solution, but it was hardly elegant. A solution that doesn't use hypergeometric functions in the middle of the solution would be nice.

$$int_{-infty}^inftyprod_{k=1}^noperatorname{sinc}{thetaover(2k-1)}dtheta=pi ,$$provided $nin{1 ... 7}$ ... for $ngeq8$, it starts being $<π$ by the most miniscule amounts!

I like

$$int_{-infty}^{infty } frac{r log left(frac{frac{frac{D^2}{4}+r^2}{D r}+1}{frac{frac{D^2}{4}+r^2}{D r}-1}right)}{frac{D^2}{4}+r^2} , dr=pi^2$$

where $D>0$ (no proof supplied).

If you make the mistake of trying to convert the $log$ term to its series form, to attempt to integrate term by term, this integral becomes really crazy, an infinite almost fractal cascade of further self similar integrals with the series for $pi/2$ gradually appearing out of the fog

$$1+frac{1}{3}left(frac{1}{2}right)+frac{1}{5}left(frac{1}{2}frac{3}{4}right)+frac{1}{7}left(frac{1}{2}frac{3}{4}frac{5}{6}right)+...=frac{pi}{2}$$

You miss all this underlying structure sensibly driving via the mathematical motorway.

This might not be a difficult integral but it made me come up with a new method to solve it so I think it's quite exotic.

Let’s do the general integral
$displaystyle I(a,b)=int_{0}^{infty}e^{-(ax^{-2}+bx^{2})}dx$

Differentiate with respect to a

$displaystyle frac{partial I}{partial a}=int_{0}^{infty}x^{-2}e^{-(ax^{-2}+bx^{2})}dx$

Now differentiate with respect to b
$displaystyle frac{partial^2 I}{partial a partial b}=int_{0}^{infty}x^{-2}x^{2}e^{-(ax^{-2}+bx^{2})}dx$

$displaystyle frac{partial^2 I}{partial a partial b}=int_{0}^{infty}e^{-(ax^{-2}+bx^{2})}dx$

$displaystyle frac{partial^2 I}{partial a partial b}=I$

Thus our integral satisfies this PDE.This is a hyperbolic homogenous PDE. It is a second order PDE but it is first order with respect to each of the variables so we’ll need two boundary conditions to determine a unique solution.(In this case two asympotic BCs and one Drichlet boundary condition will be used).Keep this in mind we’ll need it later.

Let’s complete the square of expression in the exponential.

$displaystyle I(a,b)=int_{0}^{infty}e^{-(ax^{-2}+bx^{2}-2sqrt{ab}+2sqrt{ab})}dx$

$displaystyle I(a,b)=int_{0}^{infty}e^{-(sqrt{a}x^{-1}-sqrt{b}x)^{2}-2sqrt{ab}}dx$

$displaystyle I(a,b)=e^{-2sqrt{ab}}int_{0}^{infty}e^{-(sqrt{a}x^{-1}-sqrt{b}x)^{2}}dx$

Now let’s explore more of it’s properties.One thing to note is that this integral diverges(blows up) at b=0 but at a=0 it has a well known value. It is the Gaussian integral so

$displaystyle I(0,b)=int_{0}^{infty}e^{-(bx^{2})}dx=frac{1}{2}sqrt{frac{pi}{b}}$

The negative exponential was extracted from the integral rather than the positive one beacause

$displaystyle lim_{atoinfty}int_{0}^{infty}e^{-(ax^{-2}+bx^{2})}dx=0$

and

$displaystyle lim_{atoinfty}e^{-2sqrt{ab}}=0$

So let’s assume that we assume that the solution to our PDE is of the form

$displaystyle I(a,b)=e^{-2sqrt{ab}}K(b)$

where K is a function of b(and diverges at b=0)

Let’s put this in the PDE

$displaystyle frac{partial I}{partial a}=-sqrt{frac{b}{a}}e^{-2sqrt{ab}}K(b)$

$displaystyle frac{partial^2 I}{partial a partial b}=-sqrt{frac{b}{a}}e^{-2sqrt{ab}}K^{'}(b)-frac{1}{2sqrt{ab}}e^{-2sqrt{ab}}K(b)+sqrt{frac{b}{a}}sqrt{frac{a}{b}}e^{-2sqrt{ab}}K(b)$

$displaystyle frac{partial^2 I}{partial a partial b}=e^{-2sqrt{ab}}(-sqrt{frac{b}{a}}K^{'}(b)-frac{K(b)}{2sqrt{ab}}+K(b))$

As
$displaystyle frac{partial^2 I}{partial a partial b}=I$

So

$displaystyle e^{-2sqrt{ab}}(-sqrt{frac{b}{a}}K^{'}(b)-frac{K(b)}{2sqrt{ab}}+K(b))=e^{-2sqrt{ab}}K(b)$

$displaystyle -sqrt{frac{b}{a}}K^{'}(b)-frac{K(b)}{2sqrt{ab}}+K(b)=K(b)$

$displaystyle -sqrt{frac{b}{a}}K^{'}(a)=frac{K(b)}{2sqrt{ab}}$

$displaystyle K^{'}(b)=-frac{K(b)}{2b}$

This is a separable ODE.Let’s solve it

$displaystyle frac{1}{K}dK=-frac{1}{2}frac{1}{b}db$

Let’s integrate

$displaystyle int frac{1}{K}dK=-frac{1}{2}int frac{1}{b}db$

$displaystyle ln(K)=-frac{1}{2}ln(b)+C$

$displaystyle ln(K)=ln(b^{-frac{1}{2}})+C$

$displaystyle K=e^{C}b^{-frac{1}{2}}$

Let
$displaystyle v=e^{C}$

So

$displaystyle K(b)=vb^{-frac{1}{2}}$

Thus the solution is
$displaystyle I(a,b)=ve^{-2sqrt{ab}}b^{-frac{1}{2}}$

This expression diverges at b=0 which is exactly what we wanted. Now let’s determine the constant v. As

$displaystyle I(0,b)=frac{1}{2}sqrt{frac{pi}{b}}$

So
$displaystyle frac{1}{2}sqrt{frac{pi}{b}}=vb^{-frac{1}{2}}e^{0}$
$v=frac{sqrt{pi}}{2}$

Thus the integral is

$displaystyle boxed{I(a,b)=frac{1}{2}sqrt{frac{pi}{b}}e^{-2sqrt{ab}}} (0leqslant a,b)$