Challenge Math Challenge - July 2019

[fresh_42]

Questions

1. (solved by @Flatlanderr , solved by @lriuui0x0 ) Show that $\frac{\pi}{4} + \frac{3}{25} \lt \arctan \frac{4}{3} \lt \frac{\pi}{4} + \frac{1}{6}$

2. (solved by @nuuskur ) Show that the equation $x + x^3 + x^5 + x^7 = {c_1}^2 (c_1 - x) + {c_2}^2 (c_2 - x)$ where $c_1, c_2 \in \mathbb{R}$, has only one real root.

3. (solved by @KnotTheorist ) Let $A = \begin{bmatrix} 7 & 3\\ 3 & -1 \end{bmatrix}$. Find an orthogonal matrix $P$ such that $D = P^{-1} A P$ is diagonal.

4. (solved by @nuuskur ) Using Calculus, show that the roots of the equation $x^5 + ax^4 + bx^3 + cx^2 + dx + e = 0$ cannot be all real if $2a^2 \lt 5b$

5. Consider the projective space $P^m(F_q) = P(F_q^{m+1})$. How many (t+1)-dimensional spaces in this projective space contain a given t-dimensional space?

6. (solved by @nuuskur ) Let $C$ be a subspace of $F_q^n$. We call $C$ cyclic if $C$ contains its right shifts, i.e. $x = (c_1, \dots, c_n) \in C \implies \vec{x} = (c_n, c_1 \dots, c_{n-1}) \in C$. Show that there is a correspondence between the cyclic subspaces of $F_q^n$ and the ideals of the quotient ring $F_q[X]/(X^n-1)$.

7. (solved by @nuuskur ) Let $(W_t)_{t\geq 0}$ be a Brownian motion on a suitable probability space. Put $X:= \int_{0}^t W_s^2 ds$. Calculate $E(X).$ (changed, 7/5/19)

8. (solved by @nuuskur ) Calculate the Galois group of the splitting field of $x^4 - 2x^2 -2$ over $\mathbb{Q}$. Determine the isomorph type of the Galois group.

9. Prove that every symmetric and positive definite matrix $A\in \mathbb{M}(n,\mathbb{R})$ can be uniquely written as $A=L\cdot L^\tau$, where $L$ is a lower triangular matrix with positive diagonal elements.
Calculate $L$ for $A=\begin{bmatrix}4 &2& 4& 4\\ 2& 10& 17& 11\\ 4& 17& 33& 29\\ 4& 11& 29& 39\end{bmatrix}$.

10. (solved by @nuuskur ) Let $L\subseteq H$ be a nonempty, closed, and convex set in a Hilbert space. Prove that there is an element of minimal norm in $L$.

11. (solved by @Physics lover ) Solve the following integral
$$I = \int_0^{\pi/2} \frac{\sin^{2019} x}{\cos^{2019} x + \sin^{2019} x}dx$$

12. (solved by @Pi-is-3 ) Is $N:=21^{39}+39^{21}$ divisible by $45$? Why, why not?

13. (solved by @Pi-is-3 ) Let $0<u,v,w<1$. Show that among the numbers $u(1-v)\, , \,v(1-w)\, , \,w(1-u)$ is at least one value not greater than $\dfrac{1}{4}\,.$

14. (solved by @JosephFG ) What is the ratio between the red and the blue area where $P$ and $Q$ are one fifth away from the vertices?

15. (solved by @Pi-is-3 ) In what ratio does the circumference of the circle divide the left and right sides of the square?

 

[KnotTheorist]

My Solution to 3

Spoiler: Spoiler to 3

This is just a simple eigenvalue problem, where P is just the matrix of eigenvectors, which if the eigenvectors themselves are normalized, will become an orthogonal matrix. And then it will diagonalize to its eigenvalues.
We can get that the eigenvalues of A are 8 and -2, then solving for the eigenvectors, ensuring that they are normalized, we get v1 = 1/√10 (3 1) and v2 = 1/√10 (-1 3).
P =
\begin{pmatrix}
3/√10 & -1/√10 \\
1/√10 & 3/√10
\end{pmatrix}

And then P^-1 = P^T. Then solving for D, we get D = P^-1 A P = P^T A P =
\begin{pmatrix}
8 & 0 \\
0 & -2
\end{pmatrix}

 

[lriuui0x0]

Question 1:

Spoiler: Solution

$\frac{\pi}{4} = \arctan 1$

Therefore we get $\frac{3}{25} < \arctan \frac{4}{3} -\arctan 1 < \frac{1}{6}$.

Since

$$
\begin{aligned}
\tan (a - b) &= \frac{\tan a - \tan b}{1 + \tan a \tan b} \\
a - b &= \arctan \frac{\tan a - \tan b}{1 + \tan a \tan b}
\end{aligned}
$$

Let $a = \arctan x$ and $b = \arctan y$,

we have

$$
\arctan x - \arctan y = \arctan \frac{a - b}{1 + ab}
$$

Therefore we need to prove $\frac{3}{25} < \arctan \frac{1}{7} < \frac{1}{6}$.

Expand $\arctan x$ with power series, we have $\arctan x = \sum (-1)^k \frac{x^{2k + 1}}{2k + 1}\; (k \ge 0)$. Denote partial sum as $S_k$, since the power series is an alternating series whose absolute term value is monotonically decreasing, we can prove the greatest partial sum is $S_0$, the lowest partial sum is $S_1$. Therefore $x - \frac{x^3}{3} < \arctan x < x$. Put in value $\frac{1}{7}$, we get the inequality.

To prove $S_0$ and $S_1$ are maximum and minimum partial sum respectively, first note that $S_{2k} > S_{2k + 1}$, then note $S_{2k} > S_{2k + 2}$ and $S_{2k + 1} < S_{2k + 3}$.

 

[QuantumQuest]

Let $a = \arctan x$ and $b = \arctan y$,

we have

​

$\arctan x - \arctan y = \arctan \frac{a - b}{1 + ab}$

How did you get this?

 

[lriuui0x0]

Hi QuantumQuest,

So given $a=\arctan x$ and $b=\arctan y$, put $a$ and $b$ into previous equation, we get

$$
\begin{aligned}
a − b &= \arctan \frac{\tan a − \tan b}{1+ \tan a \tan b} \\
\arctan x - \arctan y &= \arctan \frac{\tan \arctan x - \tan \arctan y}{1 + (\tan \arctan x)(\tan \arctan y)} \\
\arctan x - \arctan y &= \arctan \frac{x - y}{1 + xy}

\end{aligned}
$$

 

[nuuskur]

Some ideas for the second problem.

Spoiler: Problem 2 - incomplete

Replace c_1 =: u, c_2 =: v. Assume there is a (real) root r for the polynomial. Transform initial problem to comparison of coefficients as follows.
<br /> \begin{align*}<br /> &amp;x^7 + x^5 + x^3 + (1+u^2 + v^2)x - u^3 - v^3 \\ = &amp;(x-r)(a_6x^6 + a_5x^5 + a_4x^4 + a_3x^3 + a_2x^2 + a_1x + a_0)<br /> \end{align*}<br />
It is now a matter of finding what r is in terms of the parameters u,v. If there is no branching, then r is necessarely unique.

The coefficients are restricted as follows.
<br /> \begin{align*}<br /> &amp;a_6 = 1 \\<br /> &amp;a_5 = a_6r <br /> &amp;a_4 = a_5r + 1 \\<br /> &amp;a_3 = a_4r <br /> &amp;a_2 = a_3r + 1 \\<br /> &amp;a_1 = a_2r <br /> &amp;a_0 - a_1r = 1+u^2 + v^2 \\<br /> &amp;a_0r = u^3 + v^3<br /> \end{align*}<br />
Using
<br /> \begin{cases}a_0r = u^3 + v^3 \\ a_0 - a_1r = 1+ u^2+v^2 \end{cases}<br />
multiplying both sides of the second equality by r we get
<br /> a_0r - a_1r^2 = (1+u^2+v^2)r \iff a_1r^2 + (1+u^2+v^2)r - (u^3+v^3) = 0. <br />
For r to be unique, we require the discriminant
<br /> (1+u^2+v^2)^2 + 4a_1(u^3+v^3) = 0.<br />
By tracing the restrictions set for the coefficients a_0,\ldots , a_6, we arrive at
<br /> a_1 = r^5 + r^3 + r<br />
I predict some sort of guesswork ahead..

Not enough brainpower left at the moment. To be continued..

 

[Pi-is-3]

For problem 2
Sorry for bad latex.

Spoiler: Problem 2

The equation can be rearranged to be-
<br /> f(x)= x^7 + x^5 + x^3 + (c_1^2+c_2^2)x-(c_1^3+c_2^3)=0<br />
By Descartes rule of signs, this problem is trivialized.

<br /> Let (<div style="text-align: left"><span style="font-size: 16px"><span style="font-family: 'Noto Sans'">c_1^2+c_2^2)=k, k \ge 0<br /> Let <div style="text-align: left"><span style="font-size: 16px"><span style="font-family: 'Noto Sans'">(c_1^3+c_2^3)=t <br /> <span style="font-size: 16px"><span style="font-family: 'Noto Sans'">[\tex]<br /> <br /> <span style="font-size: 16px"><span style="font-family: 'Noto Sans'">If t&gt;0, f(x) has exactly one real root, by Descartes rool of signs. If t&lt;0, then f(-x) has exactly one real root, which is the same thing.</span></span></span></span></span></span>&#8203;</div></span></span>&#8203;</div><span style="font-size: 16px"><span style="font-family: 'Noto Sans'"><span style="font-size: 16px"><span style="font-family: 'Noto Sans'"><span style="font-size: 16px"><span style="font-family: 'Noto Sans'"><span style="font-size: 16px"><span style="font-family: 'Noto Sans'"><br /> <br /> <a href="https://en.wikipedia.org/wiki/Descartes%27_rule_of_signs" target="_blank" class="link link--external" rel="nofollow ugc noopener">https://en.wikipedia.org/wiki/Descartes&#039;_rule_of_signs</a></span></span></span></span></span></span></span></span>

 

[Pi-is-3]

Can I also contribute to maths challenge from next time?

 

[Pi-is-3]

Question 12 is trivial

Spoiler: Q 12

N:=213$$21^{39}+39^{21} (mod 9)=0$$

$$21^{39}+39^{21} (mod 5)=1-1=0$$
Hence it is divisible by 45

EDIT: Alternate

Spoiler: proof for 'high schoolers'

21^39 is divisible by 9 because it has 3 raised to a power more than 2. Similarly with 39^21.

Binomial expansion of (20+1)^39= 20^39+ (39)(20^38)+...+(39)(20)+1 which leaves 1 as remainder with five. (40-1)^21 leaves 4 remainder on the other hand. Adding them up leaves no remainder.

Hence, the sum is divisible by 45.

@fresh_42 New proof added. I am in high school BTW (11th grade).

 

[fresh_42]

Can I also contribute to maths challenge from next time?

If you like send me a PM, if possible with better LaTeX ;-)

 

[fresh_42]

Question 12 is trivial

Spoiler: Q 12

N:=213$$21^{39}+39^{21} (mod 9)=0$$

$$21^{39}+39^{21} (mod 5)=1-1=0$$
Hence it is divisible by 45

We need a proof, not a computer calculation.

 

[Pi-is-3]

We need a proof, not a computer calculation.

I didn't do a computer calculation. It's simple modular arithmetic. $$a^n (mod k)= (a (mod k))^n$$ for a being integer, and n and k being a natural numbers.
https://brilliant.org/wiki/modular-arithmetic/

 

[fresh_42]

I didn't do a computer calculation. It's simple modular arithmetic. $$a^n (mod k)= (a (mod k))^n$$ for a being integer, and n and k being a natural numbers.
https://brilliant.org/wiki/modular-arithmetic/

Can you write this in a form that high schoolers can read and understand it? You do not have to convince me!

 

[QuantumQuest]

So given ...$$\begin{aligned}
a − b &= \arctan \frac{\tan a − \tan b}{1+ \tan a \tan b} \\
\arctan x - \arctan y &= \arctan \frac{\tan \arctan x - \tan \arctan y}{1 + (\tan \arctan x)(\tan \arctan y)} \\
\arctan x - \arctan y &= \arctan \frac{x - y}{1 + xy}

\end{aligned}$$

Yes, last line is different from what you wrote in post #3 and it is correct now. With this corrected your solution is correct. Well done.

 

[QuantumQuest]

Regarding Question $1$ there is a simpler solution using just Calculus I. So, anyone other than lriuui0x0 - who has already got the credit, can also take credit if he / she finds it.

 

[QuantumQuest]

For problem 2
Sorry for bad latex.

You're missing an $x$ in your $f(x)$. Also, please write in Latex - you can take a look at "INFO"$\rightarrow$ "Help"$\rightarrow$ "LaTeX Primer" (top navigation bar) if you don't know about Latex.

Moreover, the problem states that $c_1, c_2 \in \mathbb{R}$. What if they are both zero?

 

[jcloo]

Question 4 :)

 

[QuantumQuest]

Question 4 :)

You take the third derivative as I see. What theorem are you using here? How do you get to the last two lines? Also, why is the answer in the form of an image?

 

[nuuskur]

I may have overthought about problem 2.

Spoiler: Problem 2 - take 2

Fix u,v\in\mathbb R and set
<br /> f(x) = x^7 + x^5 + x^3 + (1+u^2+v^2)x - (u^3+v^3).<br />
Its derivative is
<br /> f&#039;(x) = 7x^6 + 5x^4 + 3x^2 + 1+u^2+v^2.<br />
Since f is of odd degree it necessarily has at least one real root by the intermediate value theorem. It also holds that f&#039;(x) &gt;0 for every x\in\mathbb R. Thus f is strictly increasing, hence there can only be one real root.

I think my previous idea will fail, because the solution would be given implicitly - thus there being no progress.

Spoiler: Problem 4

Proof by contrapositive. Assume the polynomial has all real roots and set
<br /> f(x) = (x-a_5)(x-a_4)(x-a_3)(x-a_2)(x-a_1)<br />
We are interested in the coefficients of x^3, x^4, we get
<br /> a = -(a_1+a_2+a_3+a_4+a_5) \\<br /> b = a_1a_2 + a_1a_3 + a_1a_4 + a_1a_5 + a_2a_3 + a_2a_4 + a_2a_5 + a_3a_4 + a_3a_5 + a_4a_5<br />
Study relationship between quantities 2a^2 and 5b. Post expansion we have
<br /> a^2 = a_1^2 + a_2^2 + a_3^2 + a_4^2 + a_5^2 + \\ 2 \lbrace a_1a_2 + a_1a_3 + a_1a_4 + a_1a_5 + a_2a_3 + a_2a_4 + a_2a_5 + a_3a_4 + a_3a_5 + a_4a_5 \rbrace =: c + 2b.<br />
Finally we have to show 2a^2 \geq 5b i.e 2(c+2b) \geq 5b i.e 2c \geq b. This is somewhat immediate, if a bit technical. We show 4c \geq 2b. Group as follows to the left side
<br /> (a_1^2 - 2a_1a_2 + a_2^2) + (a_1^2 - 2a_1a_3 + a_3^2) + (a_1^2 - 2a_1a_4 + a_4^2) + (a_1^2 - 2a_1a_5 + a_5^2)<br />
with this we have exhausted four copies of a_1^2 and the quantity above is non-negative. Proceed with
<br /> (a_2^2 - 2a_2a_3 + a_3^2) + (a_2^2 - 2a_2a_4 + a_4^2) + (a_2^2 - 2a_2a_5 + a_5^2)<br />
now all copies of a_2^2 are exhausted and the quantity remains non-negative. Proceed similarly and obtain 4c - 2b\geq 0. This concludes the proof.

I noticed that P4 required the use of calculus? What, precisely, does that mean? If my approach doesn't qualify, it's fine.

Spoiler: Problem 4 - calculus?

Again, proof by contrapositive. We could make use of the Gauss-Lucas theorem, which states that the zeros of f&#039; are convex combinations of the zeros of f. If we assumed all the roots of f are real, by Gauss Lucas, all the roots of derivatives of arbitrary order are also real. Thus, taking the third derivative
<br /> f^{(3)}(x) = 60x^2 + 24ax + 6b<br />
and because the roots are real, we have the following restriction:
<br /> (24a)^2 - 1440b \geq 0 \iff 2a^2 \geq 5b.<br />

 

[nuuskur]

Spoiler: Problem 10

If I understand this correctly, we need to show that L contains an element u which satisfies
<br /> \|u\| = \inf _{x\in L} \|x\|<br />
Making use of the parallelogram law we get for any x,a,b\in H
<br /> \|(x-a) + (x-b)\|^2 + \|(x-a)-(x-b)\|^2 = 2(\|x-a\|^2 + \|x-b\|^2)<br />
Calculating the lhs of the above and by re-arranging we get to
<br /> \|a-b\|^2 = 2\|x-a\|^2 + 2\|x-b\|^2 - 4\|x- \frac{1}{2}(a+b)\|^2\tag{E}<br />
In light of P10, we can take x=0, but we can solve this problem more generally, fix x\in H. Set l := \inf _{y\in L} \|x-y\|. If we take a,b\in L then due to convexity \frac{1}{2}(a+b) \in L, therefore
<br /> l \leq \|x-\frac{1}{2}(a+b)\|.<br />
Per definition of infimum, there exist sequences that converge to it, so pick a_n\in L, n\in\mathbb N such that
<br /> \|x-a_n\|^2 &lt; l^2 + \frac{1}{n}<br />
By (E) we then have for all m,n\in \mathbb N
<br /> \|a_n-a_m\| \leq 2\|x-a_n\|^2 + 2\|x-a_m\|^2 -4l^2 \\ \leq 2l^2 + \frac{2}{n} + 2l^2 + \frac{2}{m} - 4l^2 = \frac{2}{n} + \frac{2}{m}<br />
This mean the sequence (a_n) is Cauchy and by completeness a_n \xrightarrow[]{}u \in H. Due to closedness u\in L. Consequently
<br /> \|x-u\|^2 = \lim _{n\to\infty} \|x-a_n\|^2 = l^2.<br />
In fact, this u is unique, for if v satisfies the same demand, then by (E)
<br /> \|u-v\| \leq 2\|x-u\|^2 + 2\|x-v\|^2 -4l^2 = 2l^2 + 2l^2 - 4l^2 = 0.<br />

Geometrically, this makes sense. For instance, in case of a ball on a plane, for a fixed point in the space, one can always pick a point on the ball closest to that fixed point.
If the fixed point is contained in the ball, the closest point to it is itself, of course. If we give up convexity, the closest point need not be unique. For instance, unit sphere is not convex, there are infinitely many points at distance 1 from the origin.

 

[Flatlanderr]

Regarding Question $1$ there is a simpler solution using just Calculus I. So, anyone other than lriuui0x0 - who has already got the credit, can also take credit if he / she finds it.

Spoiler

Let $f(x) = arctan(x).$

Since $arctan(x)$ is differentiable for all x, by the Mean Value Theorem we know
$$
\frac{f(b) - f(a)}{b - a} = f'(c)
$$
for some c on the interval $a < c < b$.

$f'(x) = \frac{1}{1 + x^2}$, which is decreasing for $x > 0$, so for $0 < a < c < b$, $f'(b) < f'(c) < f'(a)$.

Substituting, we have
$$
f'(b) < \frac{f(b) - f(a)}{b - a} < f'(a)
$$
$$
\frac{1}{1 + b^2} < \frac{arctan(b) - arctan(a)}{b - a} < \frac{1}{1 + a^2}
$$
$$
\frac{b - a}{1 + b^2} < arctan(b) - arctan(a) < \frac{b - a}{1 + a^2}
$$

Let $a = 1, b = 4/3$.

$$
\frac{\frac{4}{3} - 1}{1 + (\frac{4}{3})^2} < arctan(4/3) - arctan(1) < \frac{\frac{4}{3} - 1}{1 + 1^2}
$$
$$
\frac{3}{25} < arctan(4/3) - \frac{\pi}{4} < 1/6
$$
$$
\frac{\pi}{4} + \frac{3}{25} < arctan(4/3) < \frac{\pi}{4} + \frac{1}{6}
$$

 

[Physics lover]

My solution to 11.

By the way i wanted to know whether can we solve the same integral but indefinite.If we can please provide a solution.

 

[member 587159]

My solution to 11.
View attachment 246120

By the way i wanted to know whether can we solve the same integral but indefinite.If we can please provide a solution.

Why is there +c? It is a definite integral! For the rest,you are correct. I doubt you can find an elementary primitive. This was the solution I had in mind.

 

[Physics lover]

Why is there +c? It is a definite integral! For the rest,you are correct. I doubt you can find an elementary primitive. This was the solution I had in mind.

Oh sorry for that c.Ok i will try for another method Can we solve its indefinite one?

 

[fresh_42]

Oh sorry for that c.Ok i will try for another method Can we solve its indefinite one?

Better try to learn to use MathJax (aka LaTeX). See
https://www.physicsforums.com/help/latexhelp/

 

[QuantumQuest]

I may have overthought about problem 2.

Yes indeed. The solution you give in post #19 using simple calculus is much faster and simpler. Well done.

I noticed that P4 required the use of calculus? What, precisely, does that mean? If my approach doesn't qualify, it's fine.

There are various ways that can be used but I ask for the use of calculus as it is a fast and simple approach. Your first solution for question $4$ gets unnecessarily complex but the second is fine.

 

[QuantumQuest]

Spoiler

Let $f(x) = arctan(x).$

Since $arctan(x)$ is differentiable for all x, by the Mean Value Theorem we know
$$
\frac{f(b) - f(a)}{b - a} = f'(c)
$$
for some c on the interval $a < c < b$.

$f'(x) = \frac{1}{1 + x^2}$, which is decreasing for $x > 0$, so for $0 < a < c < b$, $f'(b) < f'(c) < f'(a)$.

Substituting, we have
$$
f'(b) < \frac{f(b) - f(a)}{b - a} < f'(a)
$$
$$
\frac{1}{1 + b^2} < \frac{arctan(b) - arctan(a)}{b - a} < \frac{1}{1 + a^2}
$$
$$
\frac{b - a}{1 + b^2} < arctan(b) - arctan(a) < \frac{b - a}{1 + a^2}
$$

Let $a = 1, b = 4/3$.

$$
\frac{\frac{4}{3} - 1}{1 + (\frac{4}{3})^2} < arctan(4/3) - arctan(1) < \frac{\frac{4}{3} - 1}{1 + 1^2}
$$
$$
\frac{3}{25} < arctan(4/3) - \frac{\pi}{4} < 1/6
$$
$$
\frac{\pi}{4} + \frac{3}{25} < arctan(4/3) < \frac{\pi}{4} + \frac{1}{6}
$$

Well done @Flatlanderr

 

[LagrangeEuler]

My solution to 11.
View attachment 246120

By the way i wanted to know whether can we solve the same integral but indefinite.If we can please provide a solution.

How you change $x \to \frac{\pi}{2}-x$ without changing interval of integration?

 

[member 587159]

How you change $x \to \frac{\pi}{2}-x$ without changing interval of integration?

The new integration bounds are from $\pi/2$ to $0$ and we get a minus sign for the integral as a result of the substitution. Switching the integral bounds removes the minus sign.

 

[Physics lover]

How you change $x \to \frac{\pi}{2}-x$ without changing interval of integration?

This is a propertie of definite integral
$\int_0^a f(x) \, dx =$ $\int_0^a f(0+a-x) \, dx$

 

[fresh_42]

AM_GM-HM is https://artofproblemsolving.com/wik..._Mean-Geometric_Mean-Harmonic_mean_Inequality

Spoiler: Qestion 13

https://i.vgy.me/A8GrmA.png

Oh, come on ...!

No graphics, handwritings and even more, no uploads on external servers please!
https://www.physicsforums.com/help/latexhelp/Presentation is part of what should be learned with these riddles. To know a solution is far from being able to convince others that your solution is one.

 

[Pi-is-3]

I am having a lot of problem with latex. I am not that bad at latex in AoPS or Stack exchange but I am not able to use it properly here :(. Example, my first line of answer should be this-

By AM-GM

$$ \frac{(u+(1-v)+v+(1-w)+w+(1-u))}{6} \geq
(u(1-v)v(1-w)w(1-u))^{\frac{1}{6}$$

but the Latex is not working. What am I doing wrong?

 

[fresh_42]

I am having a lot of problem with latex. I am not that bad at latex in AoPS or Stack exchange but I am not able to use it properly here :(. Example, my first line of answer should be this-

By AM-GM

$$ \frac{(u+(1-v)+v+(1-w)+w+(1-u))}{6} \geq
(u(1-v)v(1-w)w(1-u))^{\frac{1}{6}$$

but the Latex is not working. What am I doing wrong?

This is due to your laziness. If you copy and paste instead of actually writing it, you also copy hidden control sequences as colors or fonts. This doesn't work in LaTeX here.

If it helps you, you can download a script program, e.g. AutoHotkey, which allows you to abbreviate certain key sequences by keyboard shortcuts. For example I have \frac{}{} on Ctrl+F or \ begin{bmatrix} \ end{bmatrix} on Alt+M (without the blanks).

 

[Pi-is-3]

@fresh_42 Thanks for telling my mistake! Now I can latex again

Spoiler: Final Latexed version of Q 13

By AM-GM

$$ \frac {(u)+(1-v)+(v)+(1-w)+(w)+(1-u)} {6} \geq [(u)(1-v)(v)(1-w)(w)(1-u)]^{\frac{1}{6}}$$​

Implies

$$ \frac {1} {2} \geq [(u)(1-v)(v)(1-w)(w)(1-u)]^{\frac{1}{6}}$$​

​

Squaring both sides (works in this inequality as both sides are positive)

$$ \frac {1} {4} \geq [(u)(1-v)(v)(1-w)(w)(1-u)]^{\frac{1}{3}}$$​

Now using GM-HM

$$ \frac {1} {4} \geq [(u)(1-v)(v)(1-w)(w)(1-u)]^{\frac{1}{3}} \geq \frac {3} {\frac{1}{u(1-v)}+\frac{1}{v(1-w)} + \frac{1}{w(1-u)}}$$​

Implies

$$ \frac{1}{u(1-v)} + \frac{1}{v(1-w)} + \frac{1}{w(1-u)} \geq 12 $$​

W.L.O.G assume $u(1-v) \leq v(1-w) \leq w(1-u)$

then $\frac{3}{u(1-v)} \geq 12$

implies $$\frac{1}{4} \geq u(1-v) $$

Hence Proved

 

[fresh_42]

@fresh_42 Thanks for telling my mistake! Now I can latex again

Spoiler: Final Latexed version of Q 13

By AM-GM

$$ \frac {(u)+(1-v)+(v)+(1-w)+(w)+(1-u)} {6} \geq [(u)(1-v)(v)(1-w)(w)(1-u)]^{\frac{1}{6}}$$

Implies

$$ \frac {1} {2} \geq [(u)(1-v)(v)(1-w)(w)(1-u)]^{\frac{1}{6}}$$

Squaring both sides (works in this inequality as both sides are positive)

$$ \frac {1} {4} \geq [(u)(1-v)(v)(1-w)(w)(1-u)]^{\frac{1}{3}}$$

Now using GM-HM

$$ \frac {1} {4} \geq [(u)(1-v)(v)(1-w)(w)(1-u)]^{\frac{1}{3}} \geq \frac {3} {\frac{1}{u(1-v)}+\frac{1}{v(1-w)} + \frac{1}{w(1-u)}}$$

Implies

$$ \frac{1}{u(1-v)} + \frac{1}{v(1-w)} + \frac{1}{w(1-u)} \geq 12 $$

W.L.O.G assume $u(1-v) \leq v(1-w) \leq w(1-u)$

then $\frac{3}{u(1-v)} \geq 12$

implies $$\frac{1}{4} \geq u(1-v) $$

Hence Proved

Well done and far better written than your first attempts, and I do not mean LaTeX, I mean the structure of your proof!

You should consider to download this little helper. It saves so much time, that I can almost type at a normal speed without all these special characters. I even use keys for \alpha , \beta, \omega etc.

If anyone still wants to try: there is another proof using a binomial formula.

 

[BWV]

#7

Spoiler

Isnt #7 just the variance of W?
Var(W)=E[W^2]-E^2[W]
E[W]=E^2[W]=0
So
var(w)=E[W^2]=t

 

[fresh_42]

Well, from science advisors and such experienced members of PF as you are we can certainly expect that they can solve problems meant for the kids still in school.

 

[hilbert2]

For anyone learning proof writing and presentation skills in mathematics, it's important to remember that a proof never becomes any better by making it look like something esoteric is taking place there. Most "normal" people won't understand it anyway, even when it's made "as simple as possible but no simpler" (a quote from Einstein). When a professional mathematician writes a new long proof of some theorem, it's difficult enough to follow the reasoning in it even when it contains all the necessary detail and the reader is a professional of equal standing.

 

[nuuskur]

Great, I am not normal

 

[Pi-is-3]

Spoiler: Q15

Set $A=1+i, B=1-i, C=-1-i, D=-1+i$ as the vertices of the square.
Our circle passes through $A, D$ and $\frac{C+B}{2}=M=-i$
The center lies on the intersection of y-axis and perpendicular bisector of $DM$.
equating slopes (or equations lines, depends on you), we get the center $K= \frac{i}{4}$
Using distance formula we find circle intersects the side DC at point $L=(-1,-0.5)$.
So we get, the circle divides the side DC into the ratio 3:1

 

[member 587159]

#7

Spoiler

Isnt #7 just the variance of W?
Var(W)=E[W^2]-E^2[W]
E[W]=E^2[W]=0
So
var(w)=E[W^2]=t

It is not. Why do you think so?

Note that the question is changed to calculating $\mathbb{E}[X]$, which is a lot easier.

 

[nuuskur]

I know P13 was solved, but I like the exercise so I tried something, too.

Spoiler: Problem 13

For every 0&lt;x&lt;1 it holds x(1-x)\leq 1/4 (find the vertex of the parabola). Assume u(1-v) &gt; 1/4 and v(1-w) &gt; 1/4. Need to show w(1-u) \leq 1/4.

If 1-u \leq 1-w, then
w(1-u) \leq w(1-w) \leq 1/4.

Assume 1-u&gt;1-w. From
<br /> v(1-u) &gt; v(1-w) &gt; 1/4 \geq w(1-w)<br />
we further obtain u&lt;w&lt;v. But now
<br /> 1/4 &lt; u(1-v) &lt; u(1-u) \leq 1/4,<br />
which is impossible. Thus 1-u \leq 1-w must hold.

 

[lpetrich]

Problem 9

Spoiler

I recognize this problem as Cholesky decomposition - Wikipedia

The proof of uniqueness is by construction, and this construction uses the Cholesky-Banachiewicz-Crout algorithm, as it might be called. One must find lower-triangular matrix L for matrix A such that they suffer $\mathbf{A} = \mathbf{L} \mathbf{L}^T$. It is easy to show that taking the transpose of both sides yields this equation again. Here is that algorithm, for A having size n*n:

For j = 1 to n do:
$$ L_{jj} = \sqrt{ A_{jj} - \sum_{k=1}^{j-1} L_{jk}^2 } $$
For i = (j+1) to n do:
$$ L_{ij} = \frac{1}{L_{jj}} \left( A_{ij} - \sum_{k=1}^{j-1} L_{ik} L_{jk} \right) $$
Next i
Next j

Each new L component depends only on an A component and on previously-calculated L components, so after one pass, the calculation is complete.

I must also calculate L for
$$ A = \begin{pmatrix} 4 & 2 & 4 & 4 \\ 2 & 10 & 17 & 11 \\ 4 & 17 & 33 & 29 \\ 4 & 11 & 29 & 39 \end{pmatrix} $$

I used Mathematica's CholeskyDecomposition[] function and took the result's transpose, since that function calculates an upper triangular matrix. I then verified that that result is correct. It is
$$ L = \begin{pmatrix} 2 & 0 & 0 & 0 \\ 1 & 3 & 0 & 0 \\ 2 & 5 & 2 & 0 \\ 2 & 3 & 5 & 1 \end{pmatrix} $$

 

[lpetrich]

Problem 8, partial solution

Spoiler

It is necessary to find the Galois group for the splitting field of $x^4 - 2x^2 - 2$ over Q.

The roots of that equation are $\pm \sqrt{1 \pm \sqrt{3} }$. Their four symmetry operations are (identity), (reverse outer square-root sign), (reverse inner square-root sign), and (reverse both square-root signs). The group of these operations is rather obviously $Z_2 \times Z_2$.

 

[member 587159]

Problem 8, partial solution

Spoiler

It is necessary to find the Galois group for the splitting field of $x^4 - 2x^2 - 2$ over Q.

The roots of that equation are $\pm \sqrt{1 \pm \sqrt{3} }$. Their four symmetry operations are (identity), (reverse outer square-root sign), (reverse inner square-root sign), and (reverse both square-root signs). The group of these operations is rather obviously $Z_2 \times Z_2$.

You are right about the roots, but that's about it. The Galoisgroup is not the Klein-group, nor has it order 4.

 

[ArcHorizon]

HI! I just came across those questions and thought if it would be okay if I could send you some math questions for the August math challenge 2019!

 

[nuuskur]

Not really my area, but here goes nothing. Based on Brownian motion characterisation under section titled "Mathematics"

Spoiler: Problem 7

Put X := \int _0^t W_s^2 ds. Based on conditions 1 and 4 of the characterisation we have W_s \sim \mathcal N(0,s), where s\geq 0. Therefore
<br /> s = \mbox{var}(W_s) = \mathbb E (W_s - \mathbb EW_s)^2 = \mathbb E(W_s^2).<br />
Fubini allows us to change order of integration, so we get
<br /> \mathbb EX = \mathbb E \left ( \int _0^t W_s^2ds\right ) = \int _0^t \mathbb E(W_s^2)ds = \int_0^t sds = \frac{t^2}{2}<br />
Initially, P7 asked for variance, which turns out to be
<br /> \mbox{var}(X) = \mathbb E (X - \mathbb EX)^2 = \mathbb E\left (X - \frac{t^2}{2}\right )^2 = \mathbb E(X^2) - \frac{t^4}{4}<br />
So we need to calculate the second moment of X. I'm not sure at the moment what's happening here.

 

[member 587159]

Not really my area, but here goes nothing. Based on Brownian motion characterisation under section titled "Mathematics"

Spoiler: Problem 7

Put X := \int _0^t W_s^2 ds. Based on conditions 1 and 4 of the characterisation we have W_s \sim \mathcal N(0,s), where s\geq 0. Therefore
<br /> s = \mbox{var}(W_s) = \mathbb E (W_s - \mathbb EW_s)^2 = \mathbb E(W_s^2).<br />
Fubini allows us to change order of integration, so we get
<br /> \mathbb EX = \mathbb E \left ( \int _0^t W_s^2ds\right ) = \int _0^t \mathbb E(W_s^2)ds = \int_0^t sds = \frac{t^2}{2}<br />
Initially, P7 asked for variance, which turns out to be
<br /> \mbox{var}(X) = \mathbb E (X - \mathbb EX)^2 = \mathbb E\left (X - \frac{t^2}{2}\right )^2 = \mathbb E(X^2) - \frac{t^4}{4}<br />
So we need to calculate the second moment of X. I'm not sure at the moment what's happening here.

Correct! It should be noted that is non-trivial that the map $(t,\omega) \mapsto W_t(\omega)$ is jointly measurable (i.e. measurable w.r.t. the product sigma algebra), which is necessary to apply Fubini. But you could use this without a proof.

The calculation of $\mathbb{E}[X^2]$ is harder. If you want to attempt it anyway, here is a hint to get you (or someone else who wants to try) started:

$$X^2 = \left(\int_0^t W_u^2 du\right) \left(\int_0^t W_v^2 dv\right)=\int_0^t \int_0^t W_u^2 W_v^2 dudv$$

Now, taking expectations on both sides you can use Fubini on a triple integral and the problem reduces to finding $\mathbb{E}[W_u^2 W_v^2]$.

 

[nuuskur]

It gets weird, there has to be some kind of algebraic trick involved, which I can't think of.

Spoiler: Problem 7 - epilogue

We could try
<br /> \mathbb E(W_uW_v)^2 = \mbox{var}(W_uW_v) + \mathbb E^2 (W_uW_v)<br />
This brings more complications, though. By linearity we get
<br /> v\leq u \implies u-v=\mbox{var}(W_u-W_v) = \mathbb E(W_u-W_v)^2 = u - 2\mathbb E(W_uW_v) + v<br />
from which
<br /> v\leq u \implies \mathbb E(W_uW_v) = v<br />
not sure how that's helpful, though.

 

[member 587159]

It gets weird, there has to be some kind of algebraic trick involved, which I can't think of.

Spoiler: Problem 7 - epilogue

We could try
<br /> \mathbb E(W_uW_v)^2 = \mbox{var}(W_uW_v) + \mathbb E^2 (W_uW_v)<br />
This brings more complications, though. By linearity we get
<br /> v\leq u \implies u-v=\mbox{var}(W_u-W_v) = \mathbb E(W_u-W_v)^2 = u - 2\mathbb E(W_uW_v) + v<br />
from which
<br /> v\leq u \implies \mathbb E(W_uW_v) = v<br />
not sure how that's helpful, though.

Your attempt shows in fact that $E(W_u W_v) = \min\{u,v\}$, which is correct. But you don't need it here. What you need is $E(W_u^2 W_v^2)$. Here is a hint:

Suppose $u \geq v$. Then

$$E(W_u^2 W_v^2) = E[(\{W_u-W_v\}+W_v)^2 W_v^2]$$

and $W_u- W_v, W_v$ are independent variables. What do you know about the expectation of the product of two independent random variables?