Looking for an interesting problem/riddle involving triple integrals. - MathOverflow

Looking for an interesting problem/riddle involving triple integrals.

2010-06-02T17:30:22Z

Does anyone know some good problem in real analysis, the solution of which involves triple integrals, and which is suitable for second semester Analysis students?

Thanks!

Answer by Xandi Tuni for Looking for an interesting problem/riddle involving triple integrals.

2010-06-02T18:38:47Z

I have seen students close to suicide over the following one: For $i=1,2,3$ denote by $C_i$ be the solid cylinder in $\mathbb R^3$ with axis given by the standard basis vector $x_i$ and radius 1. Compute the volume of $C_1\cap C_2 \cap C_3$.

Answer by Andrey Rekalo for Looking for an interesting problem/riddle involving triple integrals.

2010-06-02T18:56:24Z

This is one of my all time favourites (quoted from Problems and Theorems in Analysis by Polya and Szego).

The 3D domain $\mathcal D$ is defined by the inequalities $$-1\leq x,y,z\leq 1,\quad -\sigma\leq x+y+z\leq \sigma.$$ Show that the volume of $\mathcal D$ is $$\iiint\limits_{\mathcal D}dx\, dy\, dz=\frac{8}{\pi}\int\limits_{-\infty}^{\infty} \left(\frac{\sin t}{t}\right)^3\frac{\sin \sigma t}{t} dt.$$

Hint. Show first that the number of integer lattice points that satisfy the conditions $$-n\leq x,y,z\leq n,\quad -s\leq x+y+z\leq s$$ for some $n$, $s\in\mathbb N$, is equal to $$\frac{1}{2\pi}\int\limits_{-\pi}^{\pi}\left(\frac{\sin \frac{2n+1}{2}t}{sin\frac {t}{2}}\right)^3\frac{\sin \frac{2s+1}{2}t}{\sin \frac{t}{2}} dt. $$

Edit. Check out also the lecture notes by Oliver Knill for some classical examples of volume computation and fabulous illustrations.

Answer by Charles Matthews for Looking for an interesting problem/riddle involving triple integrals.

2010-06-02T19:12:36Z

Coffee mug problem: the volume of the part of a cylinder cut off by a plane, representing the volume of the coffee you have left when you first see the bottom of the mug. (Plane is horizontal, cylinder tilted, plane touches the two circles bounding the cylinder). Much integration.

Answer by Will Jagy for Looking for an interesting problem/riddle involving triple integrals.

2010-06-02T20:42:52Z

I do not know the exact location in his Collected Works but Dirichlet found the $n$-volume of

$$ x_1, x_2, \ldots, x_n \geq 0 $$ and $$ x_1^{a_1} + x_2^{a_2} + \ldots + x_n^{a_n} \leq 1 .$$

For example with $n=3$ the volume is $$ \frac{ \Gamma \left( 1 + \frac{1}{a_1} \right) \Gamma \left( 1 + \frac{1}{a_2} \right) \Gamma \left( 1 + \frac{1}{a_3} \right) }{ \Gamma \left(1 + \frac{1}{a_1} + \frac{1}{a_2} + \frac{1}{a_3} \right) } $$

Note that this has some attractive features. The limit as $ a_n \rightarrow \infty $ is just the expression in dimension $n-1,$ exactly what we want. Also, we quickly get the volume of the positive "orthant" of the unit $n$-ball by setting all $a_j = 2,$ and this immediately gives the volume of the entire unit $n$-ball, abbreviated as $$ \frac{\pi^{n/2}}{(n/2)!} $$

I think he also exactly evaluated the integral of any monomial $$ x_1^{b_1} x_2^{b_2} \cdots x_n^{b_n} $$ on the same set.

So the question would be: given, say, positive integers $a,b,c,$ find the volume of $x,y,z \geq 0$ and $ x^a + y^b + z^c \leq 1.$ If you like, fix the exponents, the triple $a=2, b=3, c=6$ comes up in a book by R.C.Vaughan called "The Hardy-Littlewood Method," page 146 in the second edition, where he assumes the reader knows this calculation.

This came back to mind because of a recent closed question on the area of $x^4 + y^4 \leq 1.$

Answer by J. H. S. for Looking for an interesting problem/riddle involving triple integrals.

2010-06-02T21:05:35Z

I guess you can find plenty of them in the Problem Department of the American Mathematical Monthly. For instance, here is a nice one by M. Hajja and P. Walker:

Evaluate $\displaystyle \int_{0}^{1}\int_{0}^{1} \int_{0}^{1} (1+u^{2}+v^{2}+w^{2})^{-2} du dv dw $.

Good thing about this one is that there may exist a CAS out there that does not give you the answer at once.

Answer by engelbrekt for Looking for an interesting problem/riddle involving triple integrals.

2010-06-02T22:16:44Z

Ask the students what is the average distance from a point in the ball of radius R to the center of the ball. You will get the answer R/2. You then point out that the volume within R/2 of the center is much smaller than the remaining volume. Next some bright student proposes the radius that equates the volume inside and outside the shell. You explain why that does not give the correct answer either, which is more subtle. You commend the student who equated volumes for a good proposal, and ask if anyone has a better one. Then it is time to set up the very simple triple integral that gives the answer, and solve it by the method of spherical shells, or by spherical coordinates. And you compare the correct answer with the answer obtained by equating volumes, and see that they are quite close.

The emphasis in this example is more on setting up the right triple integral to answer the question, and not so much on evaluating a complicated integral.

You should let the students have a little time to ponder. I let them have about three minutes for each stage, because I don't want to lose the slower ones.

Answer by Wadim Zudilin for Looking for an interesting problem/riddle involving triple integrals.

2010-06-02T23:08:19Z

The students can do $$ \iiint_{[0,1]^3}\frac{x^n(1-x)^ny^n(1-y)^nz^n(1-z)^n}{(1-(1-xy)z)^{n+1}}\ dx\ dy\ dz, $$ at least for $n=0$ and $n=1$. These are the integrals producing Apery's approximations to $\zeta(3)$; the original paper is [F. Beukers, Bull. London Math. Soc. 11 (1979) 268--272].

Answer by David Hansen for Looking for an interesting problem/riddle involving triple integrals.

2010-06-03T03:25:40Z

Not suitable for any student, but I can't resist giving it:

$\int_{0}^{\pi} \int_{0}^{\pi} \int_{0}^{\pi} \frac{dxdydz}{3-\cos{x}-\cos{y}-\cos{z}} = \frac{\sqrt{6}}{96}\Gamma(\tfrac{1}{24})\Gamma(\tfrac{5}{24})\Gamma(\tfrac{7}{24})\Gamma(\tfrac{11}{24})$,

proven by G. N. Watson in 1939.

Answer by David Bar Moshe for Looking for an interesting problem/riddle involving triple integrals.

2010-06-03T14:25:04Z

The volume of the real flag manifold $Fl_R^3 = O(3)/Z_2^3$ can be obtained on one hand by the explicit integration on the $O(N)$ invariant volume element on its big cell:

$\int_{-\infty}^{\infty}dx_1\int_{-\infty}^{\infty}dx_2\int_{-\infty}^{\infty}dx_3(1+x_1^2+(x_3-\frac{x_1x_2}{2})^2)^{-1} (1+x_2^2+(x_3+\frac{x_1x_2}{2})^2)^{-1}$

This integral is not hard to evaluate using elementary integration techniques. The result $2\pi^2$ can also be obtained from: $Vol(Fl_R^3)= Vol(RP^1) Vol(RP^2) = \frac{Vol(S^1)}{2}\frac{Vol(S^2)}{2}$

Answer by Andrey Rekalo for Looking for an interesting problem/riddle involving triple integrals.

2010-06-04T00:23:08Z

Feynman's formulae for products. $$\iiint\limits_{[0,1]^3}\frac{x_1^2 x_2}{[(a_4-a_3)x_3 x_2x_1+(a_3-a_2)x_2 x_1+ (a_2-a_1)x_1+a_1]^4}dx_1dx_2dx_3=$$ $$=\int\limits_{0}^{1}dx_1\int\limits_{0}^{1-x_1}dx_2\int\limits_{0}^{1-x_1-x_2}dx_3 \frac{1}{[(a_3-a_4)x_3+(a_2-a_4)x_2+(a_1-a_n)x_1+a_4]^4}=\frac{1}{3!a_1a_2a_3a_4}.$$

There are several ways to compute the integrals, the simplest one is probably by induction in dimension (an obvious generalization of the formula holds true for any $n=\dim [0,1]^n$ with the numerator in the first integral being $x_1^{n-1}x_2^{n-2}\dots x_{n-1}$.)

Answer by Daniel Miller for Looking for an interesting problem/riddle involving triple integrals.

2010-07-06T19:58:06Z

A good example that my Honors Calc prof gave me is the following: $$ \zeta(n) = \int_{[0,1]^n}\frac{d\boldsymbol x}{1-x_1\dotsm x_n} $$ The proof is an easy induction on $n$.

Answer by Menny for Looking for an interesting problem/riddle involving triple integrals.

2010-08-10T19:29:15Z

This riddle is better phrased in term of double integral but can work for cubes instead of rectangles.

Here is the riddle: Call a rectangle integral if the length of (at least) one of its edges is an integer.

Prove that a rectangle that can be tessellate by integral rectangles if also integral rectangle.

It is a huge hint to say that it has something to do with double integral so you may want to avoid it in the beginning.