# Math Challenge - June 2020

• Challenge
• Featured
Mentor

## Summary:

Normed, Banach and Hilbert spaces, topology, geometry, linear algebra, integration, flux.
Authors: Math_QED (MQ), Infrared (IR), Wrobel (WR), fresh_42 (FR).

## Main Question or Discussion Point

Questions

1.
(solved by @nuuskur ) Let ##H_1, H_2## be Hilbert spaces and ##T: H_1 \to H_2## a linear map. Suppose that there is a linear map ##S: H_2 \to H_1## such that for all ##x\in H_2## and all ##y \in H_1## we have
$$\langle Sx,y \rangle = \langle x, Ty \rangle$$
Show that ##T## is continuous. (MQ)

2. (solved by @strangerep , @PeroK ) Let ##A## and ##B## be complex ##n\times n## matrices such that ##AB-BA## is a linear combination of ##A## and ##B##. Show that ##A## and ##B## must have a common eigenvector. (IR)

3. (solved by @zinq ) Give an example of a vector space ##X## over ##\mathbb{R}## and a convex subset ##Q\subset X,\quad Q\ne X## such that ##Q## is not contained in any half-space of ##X##. (WR)

4. (solved by @benorin , @Delta2 ) Let ##S:=\{\,(x,y,z)\in \mathbb{R}^3\,|\,x^2+y^2=(2-z)^2,\,0\leq z\leq 2\,\}## be the surface of a cone ##C## with a circular cross section and a peak at ##(0,0,2)##. The orientation of ##S## be such, that the normal vectors point outwards. Calculate the flux through ##S## of the vector field (FR)
$$F\, : \,\mathbb{R}^3 \longrightarrow \mathbb{R}^3\, , \,F(x,y,z)=\begin{pmatrix}xy^2\\x^2y\\(x^2+y^2)(1-z)\end{pmatrix}.$$

5. (solved by @nuuskur ) Let ##X## be a normed space and ##f:X\to\mathbb{R}## be a linear functional that is not bounded. Show that ##\ker f## is dense in ##X##. (WR)

6.a.) Let ##G## be a group with a compact Hausdorff topology for which multiplication is continuous. Show that ##G## is a topological group, i.e. show that inversion is continuous. (MQ)
6.b.) Is this still true if we don't assume that ##G## is compact Hausdorff? If yes, give a proof. Otherwise, present a counterexample. (MQ)

7. (solved by @StoneTemplePython ) Let ##A## be an ##n\times n## matrix such that
1. the off-diagonal entries of ##A## are all positive
2. the sum of the entries in each row is negative.
Show that ##A## is invertible. (IR)

8. (solved by @Fred Wright , @benorin ) Calculate for ##|\alpha|\geq 1##
$$\int_0^\pi \log (1-2\alpha \cos(x)+\alpha^2)\,dx$$
a.) without using series expansions
b.) by using serious expansions. (FR)

9. Let ##X## be a Banach space and let ##Y## be a normed space. There is a linear operator ##A:X\to Y##.
As usual ##X',Y'## stand for the spaces of bounded linear functionals and the dual operator ##A':Y'\to X'## is defined. Perhaps ##A'## is not bounded but it is defined on the whole ##Y'##.
Prove that ##A## is bounded. (WR)

10. (solved by @benorin ) Evaluate ##\int_0^{\infty}\frac{\log(x)}{x^2-1}dx.## (IR)

High Schoolers only

11.
(solved by @Halc , @etotheipi ) You have two nails in a wall. Can you hang a rope around them in such a way that if either nail is removed, then the rope will fall to the floor?

12.a.) (solved by @etotheipi , @Adesh ) Show that ##\sqrt{i^i} \in \mathbb{R}## where ## i ## is the imaginary unit ##i=\sqrt{-1}.##
12.b.) (solved by @Adesh ) Which of the following equation signs is wrong and why?
$$-1 \stackrel{(1)}{=}i\cdot i \stackrel{(2)}{=}\sqrt{-1}\cdot \sqrt{-1}\stackrel{(3)}{=}\sqrt{(-1)\cdot(-1)}\stackrel{(4)}{=}\sqrt{1}\stackrel{(5)}{=}1$$
12.c.) Calculate all solutions of ##z^3=1## by three different methods.

13. (solved by @Adesh , @PhysicsBoi1908 ) Which is the smallest natural number ##n\in \mathbb{N}_0## such that there are no integers ##a,b\in \mathbb{Z}## with ##3a^3+b^3=n?##

14. (solved by @etotheipi ) Is it possible to cover an equilateral triangle with two smaller equilateral triangles without a gap? It's not required that they are of equal area, nor that they won't overlap, only that they are smaller and together have a greater area than the original triangle.

15. (solved by @PhysicsBoi1908 ) Prove: Given ##n## different integers ##\{\,a_1,\ldots,a_n\,\}##, then there exists a subset ##\{\,a_{j_1},\ldots,a_{j_m}\,\}## with ##1\leq j_1 < \ldots < j_m \leq n## such that ##n## divides ##a_{j_1}+\ldots+a_{j_m}\,.##

Last edited:
PhysicsBoi1908, kith, Anti Hydrogen and 6 others

I just want to know if question number 8 requires complex analysis. Can we do it by elementary methods and other things?
(I will delete this post of mine if it is inappropriate to ask)

Question 12 a.
Let ## \sqrt{ i ^i} = z ##. Now, we have
$$i ^{ i/2} = z \\ \frac{i}{2} ln (i) = ln (z) \\$$
Since, ##e^{i\theta} = \cos \theta + i\sin \theta##, we have ## e^{i \pi/2} = i##, therefore ## ln(i) = i \pi/2 ##.

$$\frac{i}{2} ~ i \frac{\pi}{2}= ln(z) \\ -\frac{\pi}{4} = ln(z) \\ z= e ^{-\pi /4}$$
Hence, ##z## is a real number.

Delta2 and Ishika_96_sparkles
Mentor
I just want to know if question number 8 requires complex analysis. Can we do it by elementary methods and other things?
(I will delete this post of mine if it is inappropriate to ask)
It can be done with real analysis and elementary methods. However, part one is rather tricky, whereas part two only requires to find the series expansion which fits best. Here is a hint for part a):
$$\displaystyle{\int_0^{\pi/2} \log(\sin(x))\,dx=-\frac{\pi}{2}\log(2)}$$

Mentor
Question 12 a.
Let ## \sqrt{ i ^i} = z ##. Now, we have
$$i ^{ i/2} = z \\ \frac{i}{2} ln (i) = ln (z) \\$$
Since, ##e^{i\theta} = \cos \theta + i\sin \theta##, we have ## e^{i \pi/2} = i##, therefore ## ln(i) = i \pi/2 ##.

$$\frac{i}{2} ~ i \frac{\pi}{2}= ln(z) \\ -\frac{\pi}{4} = ln(z) \\ z= e ^{-\pi /4}$$
Hence, ##z## is a real number.
If you use the complex logarithm, you have to manage the branches, i.e. prove why the formulas from the reals still hold. We don't even know what ##\ln z## is without additional explanations.

Can you give a solution without complex functions? (Hint: Euler's identity.)

If you use the complex logarithm, you have to manage the branches, i.e. prove why the formulas from the reals still hold. We don't even know what lnzln⁡z\ln z is without additional explanations.
I’m sorry sir, but I couldn’t understand what you said. In the last step I just took the exponential on both sides, hence ##ln (z)## became ##z##.

Mentor
I’m sorry sir, but I couldn’t understand what you said. In the last step I just took the exponential on both sides, hence ln(z)ln(z) became zz.
Sure, but what is lnzln⁡z? It is not uniquely defined. The equation z=exz=ex has many solutions in the complex numbers.

One has to be especially careful if we apply real formulas to complex numbers, see part b.)

Question 12 b.
Equality number 3 is wrong. There we have written
$$\sqrt{-1} \cdot ~ \sqrt{-1} = \sqrt{ (-1) \cdot (-1) }$$
But this is true only when the radicand is positive. When they are negative we have to put a negative sign before the radical for combining them. Here is the proof:
$$\sqrt{-a} \cdot \sqrt {-b}= \sqrt{a} i \cdot \sqrt{b} i \\ \sqrt{-a} \cdot \sqrt{-b}= -\sqrt{a}\sqrt{b} \\ \sqrt{-a} \cdot \sqrt{-b} = - \sqrt{ab}$$
And putting a negative sign in after equality number 3 would result in a sound conclusion.

Ishika_96_sparkles
@fresh_42
For question 8th,I am getting iota (i) in my answer.Am I wrong or it is there?Can you please tell.

etotheipi
2019 Award
I’m sorry sir, but I couldn’t understand what you said. In the last step I just took the exponential on both sides, hence ##ln (z)## became ##z##.
For number 12 I would just say ##i^{\frac{i}{2}}=((e^{\frac{i\pi}{2}})^i)^{\frac{1}{2}} = (e^{-\frac{\pi}{2}})^\frac{1}{2} = e^{-\frac{\pi}{4}}##

Delta2
Mentor
Question 12 b.
Equality number 3 is wrong. There we have written
$$\sqrt{-1} \cdot ~ \sqrt{-1} = \sqrt{ (-1) \cdot (-1) }$$
But this is true only when the radicand is positive. When they are negative we have to put a negative sign before the radical for combining them. Here is the proof:
$$\sqrt{-a} \cdot \sqrt {-b}= \sqrt{a} i \cdot \sqrt{b} i \\ \sqrt{-a} \cdot \sqrt{-b}= -\sqrt{a}\sqrt{b} \\ \sqrt{-a} \cdot \sqrt{-b} = - \sqrt{ab}$$
And putting a negative sign in after equality number 3 would result in a sound conclusion.
Yes. Here is a more formal way to show it:
\begin{align*}
\sqrt{-1}\cdot \sqrt{-1}& \stackrel{\text{Gauß}}{=} \left(\cos(\pi/2)+i \sin(\pi/2)\right) \cdot \left(\cos(\pi/2)+i \sin(\pi/2)\right) \\
&\stackrel{\text{Euler}}{=}e^{i\pi/2}\cdot e^{i\pi/2}\\
&=e^{i\pi}\\
&=\cos(\pi)+i \cdot\sin(\pi)\\
&=-1+i\cdot 0\\
&=-1
\end{align*}

Mentor
@fresh_42
For question 8th,I am getting iota (i) in my answer.Am I wrong or it is there?Can you please tell.
The value of the integral is real.

Physics lover
Mentor
For number 12 I would just say ##i^{\frac{i}{2}}=((e^{\frac{i\pi}{2}})^i)^{\frac{1}{2}} = (e^{-\frac{\pi}{2}})^\frac{1}{2} = e^{-\frac{\pi}{4}}##
Yes. And with ##i=0+1\cdot i = \cos(\pi/2)+i \sin(\pi/2)=e^{i\pi /2}## it would have been perfect.

etotheipi
etotheipi
2019 Award
Yes. And with ##i=0+1\cdot i = \cos(\pi/2)+i \sin(\pi/2)=e^{i\pi /2}## it would have been perfect.
I left the rest of the proof in my username. "##i## am ##\text{e to the i pi (over two)}## "

Sure, but what is lnzln⁡z? It is not uniquely defined. The equation z=exz=ex has many solutions in the complex numbers.

One has to be especially careful if we apply real formulas to complex numbers, see part b.)
Okay, in my original solution I wrote ##ln(i)## as ##e^{i\pi/2}## so, sir, I claim that ##ln(z)## means the principle value natural logarithm of ##z##.

I left the rest of the proof in my username. "##i## am ##\text{e to the i pi (over two)}## "
Oh My God! I used to pronounce you as “Ethopia” really. I never thought of that, I used to think that you had some relations with Ethopia.

Math_QED and etotheipi
Mentor
Okay, in my original solution I wrote ##ln(i)## as ##e^{i\pi/2}## so, sir, I claim that ##ln(z)## means the principle value natural logarithm of ##z##.
But this is the problem. You used a function but didn't explain its meaning. As you have seen in your solution of part b.), one has to be cautious with complex numbers. This caution was the main reason for the question.

etotheipi
2019 Award
14. Is it possible to cover an equilateral triangle with two smaller equilateral triangles without a gap? It's not required that they are of equal area, nor that they won't overlap, only that they are smaller and together have a greater area than the original triangle.
My initial reaction was this; wherever you put it, a single smaller triangle can only cover either zero or one vertices of the larger triangle, since the separation of two vertices of the large triangle is larger than the side length of the smaller triangle. So the maximum number of vertices of the larger triangle you can cover with two smaller triangles is two; this means that at least one vertex of the large triangle remains uncovered wherever you put the other two.

wrobel
I am sorry, in problem 9 please replace "conjugated operator" with "dual operator"
Literal translation from Russian into English was a bad idea. I am sorry again

Mentor
My initial reaction was this; wherever you put it, a single smaller triangle can only cover either zero or one vertices of the larger triangle, since the separation of two vertices of the large triangle is larger than the side length of the smaller triangle. So the maximum number of vertices of the larger triangle you can cover with two smaller triangles is two; this means that at least one vertex of the large triangle remains uncovered wherever you put the other two.
Yes. The underlying principle of the reasoning is called "the pigeonhole principle". It is a major and important proof technique.

etotheipi
StoneTemplePython
Gold Member
2019 Award
7. Let ##A## be an ##n\times n## matrix such that
1. the diagonal entries of ##A## are all positive
2. the sum of the entries in each row is negative.
Show that ##A## is invertible. (IR)

##A:=\left[\begin{matrix}2 & 0 & -1 & -3\\0 & 2 & -3 & -1\\-1 & -3 & 2 & 0\\-3 & -1 & 0 & 2\end{matrix}\right]
##
##A\mathbf 1 = -2\cdot \mathbf 1##

but ##\det\big(A\big) = 0##

Infrared
Gold Member
Thanks for the the catch @StoneTemplePython Condition 1 should be "The ##\textbf{off-diagonal}## entries of ##A## are all positive." I should have checked more carefully.

benorin
Homework Helper
Gold Member
#8) a)

Let ##I(\alpha ) := \int_0^\pi \log (1-2\alpha \cos(x)+\alpha^2)\,dx\quad (\alpha \geq 1)##, then

$$\begin{gathered} \tfrac{dI}{d\alpha } = \int_0^\pi \tfrac{\partial }{\partial \alpha} \log (1-2\alpha \cos(x)+\alpha ^2)\,dx = 2 \int_0^\pi \tfrac{\alpha -\cos (x) }{1-2\alpha \cos (x)+\alpha ^2} \,dx \\ = \tfrac{1}{\alpha} \int_0^\pi\left(1- \tfrac{1-\alpha ^2}{1-2\alpha \cos(x)+\alpha^2} \right) \,dx \\ \end{gathered}$$

I used a table of integrals here and got

$$\tfrac{dI}{d\alpha } =\tfrac{\pi}{\alpha }-\tfrac{2}{\alpha}\left[ \arctan \left(\tfrac{1+\alpha}{1-\alpha}\tan \left( \tfrac{x}{2}\right)\right)\right| _{x=0}^{\pi} =\tfrac{2\pi}{\alpha}$$

hence

$$I(\alpha )=2\pi \int\tfrac{d\alpha}{\alpha}= 2\pi\log | \alpha | + C$$

I'll leave the rest of the solution (determining the constant) to somebody else. Good luck!

Note: I've read this problem somewhere before.

Summary:: Normed, Banach and Hilbert spaces, topology, geometry, linear algebra, integration, flux.
Authors: Math_QED (MQ), Infrared (IR), Wrobel (WR), fresh_42 (FR).

Prove: Given ##n## different integers {a1,…,an}{a1,…,an}\{\,a_1,\ldots,a_n\,\}, then there exists a subset {aj1,…,ajm}{aj1,…,ajm}\{\,a_{j_1},\ldots,a_{j_m}\,\} with 1≤j1<…<jm≤n1≤j1<…<jm≤n1\leq j_1 < \ldots < j_m \leq n such that nnn divides aj1+…+ajm
Although, it is clearly written “n different integers”, but then also I want to ask if all of them are different or not all of them are equal? I mean is there a possibility that two integers could be same?

My attempt at Question 15:

Proof by Induction:-

Base Case: Let ##n=1##, so we have a singleton set ##\{a_1\}## and the only subset of this is ##\{a_1\}## itself and it is of course, divisible by 1.

Let's say when we have ##k## different integers, ##\{a_1, a_2, a_3, \cdots , a_k\}## we take a subset ##\{a_{j1}, a_{j2}, a_{j3}, \cdots , a_{jm}\}## such that
$$\frac{a_{j1} + a_{j2} + \cdots + a_{jm} }{k} = q$$

Inductive Step: Let's add just one more integer to our previous set of integers, we add ##q##. Now, we have ##k+1## integers. So, we have ##\{a_1, a_2, \cdots , a_k, q\}##. Take the same subset that we took in the ##k##th case along with ##q## in it, so we have ##\{a_{j1}, a_{j2}, \cdots , a_{jm} , q\}##. Now,
$$\frac{a_{j1} + a_{j2} + \cdots a_{jm} + q}{k+1} = \frac{kq + q}{k+1} \\ \frac{a_{j1} + a_{j2} + \cdots a_{jm} + q}{k+1} = q$$

Hence, we can always choose a subset from the set of given ##n## integers such that ##n## divides the sum of members of subset.

A few words: I suspect the step when I said "let's add ##q## to our set and we will get ##k+1## integers" because I have to make sure that ##q## doesn't occur in the set ##\{a_1, a_2, \cdots a_k\}## (because we're asked in the question to prove for "##n## different integers"). This is the only thing I doubt, however, question-posters may find some other mistakes, they are welcome to inform me about the flaws in the proof.