For questions concerning a specific proof, asking for verification, identifying errors, suggestions for improvement, etc.

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1answer
99 views

How many different ways are there to organize 2 groups?

In how many different ways can you split 10 people into two groups with the same amount of people? My attempt: Since the order in which you choose someone doesn't matter, I chose to calculate the ...
0
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1answer
11 views

Give an L-formula ϕ4(x) that defines the interval [1,√2) ⊂ R in M.

Let L = {+, · P} where + and · are binary function symbols and P is a unary predicate symbol, and let M be an L-structure where its domain |M| is the set R of real numbers, + and · are the usual ...
1
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0answers
28 views

Continuous function on compact subset of $\mathbb R$ to itself has a fixed point.

Let $f:[a,b] \to [a,b]$ be continuous. Then $f$ has at least a fixed point. I read the following proof from Limaye book. Define $F(x)=f(x)-x.$ Since $a \leq f(x) \leq b,\ \quad F(a)\leq 0 \ \quad ...
2
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1answer
54 views

Can the expression $6^{2n} - 25$ be a prime for all $n \geq 2$?

Can the expression $6^{2n} - 25$ be a prime for any $n \geq 2$? My attempt to solve the problem: No, it cannot. $6^{2n} - 25 = (6^{n})^{2} - 25 = (6^{n})^{2} - 5^{2} = (6^{n} + 5)(6^{n} - 5)$ And ...
0
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4answers
61 views

For a function Y : X→X , if Y is injective, then Y∘Y∘Y is injective.

For a function Y : X→X , if Y is injective, then Y∘Y∘Y is injective. My attempt: Using contrapositive, if Y is not injective. then Y ∘ Y is not injective, the there exist x, x' ∈ X with x ≠ x' but ...
1
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1answer
31 views

Cases of $A^2 = -I$. Why is there a contradiction when reusing this proof?

I had to prove that $\nexists ~A \in M_{3,3}(\mathbb R) : A^2 = - \mathbb I.$ I argued $$\iff A=-A^{-1}$$ $$\iff \det( A)=\det(-A^{-1})$$ $$\iff \det( A)=(-1)^n\det A^{-1}$$ $$\iff \det (A) + \det ...
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2answers
47 views

Let S be the set of stars in our galaxy. There is same number subsets as functions f : S --> {a,b}? [duplicate]

Question: Let $S$ be the set of stars in our galaxy. There is exactly the same number of subsets of stars in our galaxy as there are functions $f:S \to \{a,b\}$ . My solution is; True.However, I'm ...
0
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1answer
50 views

Mathematical Induction. Horses made me question my understanding

I recently read about the false inductive proof that all horses are the same colour. There are some mathSE threads about this already (MathSE_thread_1, MathSE_thread_2). After reading this, I now ...
3
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1answer
27 views

If $q^k n^2$ is an odd perfect number with Euler factor $q^k$, can $q = 73$ hold?

Call a number $N$ perfect if $\sigma(N)=2N$ where $\sigma$ is the classical sum-of-divisors function. If $N = q^k n^2$ is an odd perfect number, can $q = 73$ hold? Here is my attempt: Since ...
3
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0answers
33 views

Order of prime ideals over split primes in the class group.

Let $P$ be a prime ideal of $\mathcal O_K$ ($K$ a quadratic field) and let $P$ have norm $p$ where $p$ is a split prime. Is it possible for the ideal class $[P]$ to have order less than three? I feel ...
3
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0answers
27 views

Does $[E:F]=|Aut(E/F)|$ imply Galois extension?

Let $E/F$ be a finite field extension such that $[E:F]=|Aut(E/F)|$. Then, is $E/F$ Galois? Even though I have proven it, I'm not sure of it. Is this really true? Here's how I proved it: Let $\bar ...
1
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4answers
84 views

There exist a set $X$ such that the number of function $y: x\to \{1,2,3\}$ is $1000$.

There exist a set $X$ such that the number of function $y: x\to \{1,2,3\}$ is $1000.$ My attempt: False, Let set $z = \{1,2,3\}$ then $|z|^{|x|}$ is set of function $y:x\to z.$ $|x| = n$ and $|z| = ...
1
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0answers
14 views

Is there any mistake? Proof related to the Poisson summation formula.

I need to prove the following statement: Let $\varphi \in C(\Bbb R)$ with compact support. Then, $$ \Big \Vert \sum_{k\in \Bbb Z} \varphi(k) e^{ikx} \Big \Vert_{L_1(0,2\pi)} \leq C \Vert ...
0
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0answers
29 views

Show $\lim_{n\to\infty}(Lu_n) = L(\lim_{n\to\infty} u_n)$

Suppose $\{u_n\}$ is a convergent sequence in Hilbert space $H$ and $L$ is a bounded (continuous) linear operator on $H$. Use the definition of convergence to show that $\lim_{n\to\infty}(Lu_n) = ...
2
votes
1answer
37 views

Are the extrema of this function global or local?

Last question about this function, I promise. The function $f: \mathbb R \rightarrow \mathbb R$ is given by $$f(x) = \begin{cases} \frac{x^2+5x+7}{x+3} & \mathrm{for} \; x < -3 \\ 0 & ...
1
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1answer
26 views

Is this proof correct for $p_1\times p_2 \mod k = p_3$?

I am trying to prove that if $\gcd(p_1, k) = \gcd(p_2, k) = 1$, then $$\gcd(r, k) = 1$$ where $r = p_1\times p_2 \mod k$. This fact is essential to guarantee that a unit group $U(n)$ of a group ...
0
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2answers
30 views

Prove that a function $f:X \to Y $ is injective if and only if $\forall x_1, x_2 \in X$ where $f(x_1) = f(x_2)$ implies that $x_1 = x_2$

Prove that a function $f:X \to Y $ is injective if and only if $\forall x_1, x_2 \in X$ where $f(x_1) = f(x_2)$ implies that $x_1 = x_2$ Taking the contrapositive we get (this is the step I'm a ...
2
votes
1answer
32 views

Why this set is the pole set of $z$?

Suppose $V\subset \mathbb P$ a projective variety, $z$ a rational function on $V$ and let's define $J_z=\{F\in k[X_1,\ldots, K_{n+1}\mid \overline Fz\in \Gamma_h(V)\}$ and $S_z$ the polo set of $z$, ...
0
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2answers
24 views

Continuous and bounded - Check my proof please

Let $f : [0, ∞) → \mathbb{R}$ be continuous such that $\lim_{x→+∞} f(x) = 0$. Prove that $f$ is bounded on $[0, ∞)$ By our hypothesis and the definition of continuity, given $ c \in [0, \infty), ...
1
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2answers
44 views

Legendre symbol, a theoretical question.

I need to show that if $p$ is a prime number of the form $p=4m+1$, then for any divisor $d$ of $m$: $$\left(\frac{d}{p} \right) = 1$$ where $\left(\frac{d}{p} \right)$ is the Legendre symbol. My ...
0
votes
1answer
33 views

Is the function continuous and differentiable at $x=-2$?

The function $f: (-3, \infty)$ is given by $$f(x) = \begin{cases} \frac{x^2+5x+7}{x+3} & \mathrm{for} \; -3 < x < -2 \\ 1 & \mathrm{for} \; x = -2 \\ -x-e^{-x}+e^2-1 & \mathrm{for} ...
1
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1answer
28 views

Proof of (a step in the proof of) the Law of Large Numbers

Theorem: Let $f:[0,1] \to \mathbb R$ be a measurable function bounded by $c$. Let $U_1,U_2,\ldots,U_n$ be i.i.d. and Uniform$(0,1)$. Then: $$ P \left( \left\lvert ...
1
vote
1answer
26 views

In $\mathscr{V}$, let $X \subset \mathscr{V}$ be a set of $n$ vectors. $Y \subset X$ contains vectors all scalar multiples, $X$ linearly dependent.

I would just like to verify that my proofs are sound and receive any suggestions on rewording. (If relevant, I am self-studying and haven't done a serious proof in about a year.) $\mathscr{V}$ is a ...
1
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1answer
22 views

Prove $\vdash\forall x(\alpha\to\beta)\to(\exists x\alpha\to\exists x\beta)$

I want to show that: $\vdash\forall x(\alpha\to\beta)\to(\exists x\alpha\to\exists x\beta)$ I started my deduction as follows: $\vdash\forall x(\alpha\to\beta)\to(\forall ...
2
votes
2answers
53 views

$x^n + y^n = z^n$, $n>1$ To show that $x,y,z$ is greater than $n$

Problem: If $x$,$y$,$z$ and $n>1$ are natural numbers with $$x^n+y^n = z^n$$ then show that x,y and z are all greater then $n$. My approach, from Fermat's Theorem we know that $x^n + y^n = z^n$ ...
0
votes
0answers
29 views

Banach Tarski Notation

Okay, I think I have a full notation and the rules of it how to extend the Banach-Tarski Paradox to an abritary number of cutoffs, as I introduced in Another way of extending the Banach-Tarski ...
3
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2answers
61 views

My proof that an n digit number, times an n digit number can be expressed as a 2n digit number

I am very proud to say this is the first time I've actually used maths to endeavour to prove something without it being related to a question from my course! Statement In a base $B$, an $n$ digit ...
0
votes
2answers
29 views

Conditional expectation of random variable

I have this home assignment in Introduction to Probability, and I'm not comfortable with definitions and heuristics. I really need someone to check if I'm even in the right direction. The question: ...
1
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1answer
34 views

Prove a limsup and liminf inequality.

Let $(a_n)_{n\in\mathbb{N}}$ be a bounded sequence and let $B_n = \frac{1}{n} \sum_{i=1}^n a_i$ for each $n \in \mathbb{N}$. Prove that $\liminf a_n \le \liminf B_n \le \limsup B_n \le \limsup a_n$. ...
4
votes
2answers
37 views

How many ways to stack $n$ boxes of 4 colours so that no $2$ blue boxes are consecutive.

Let $X_n$ denote the number of ways to stack red, white and blue and green boxes, find the ways to count the ways of stacking n boxes, with no consecutive blue boxes. My attempt: Let $X^R_n$ denote ...
2
votes
1answer
29 views

Show that if $(X,d)$ is compact then, every open covering of $X$ has a Lebesgue number.

Let $(U_i)_{i \in I}$ be an open cover of a metric space $(X,d)$, a number $\epsilon >0$ is called a Lebesgue number of $(U_i)_{i \in I}$ if for all $x \in X$ exist $j \in I$ such that ...
0
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0answers
23 views

To a given straight line in a given rectilinear angle, to apply a parallelogram equal to a given triangle.

There's again one small detail on which I'm not sure. (Proposition 44 - book 1) http://aleph0.clarku.edu/~djoyce/java/elements/bookI/propI44.html Here's the quote : "Then HLKF is a parallelogram, HK ...
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0answers
15 views

discrete mathematics matrix relation proof [on hold]

Show that if MR is the matrix representing the relation R, then M[n] R is the matrix representing the relation Rn.
2
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2answers
31 views

How to establish the distributive property of sum notation

Establish the following property of sum notation: $$\sum_{i=1}^{n}(a_i+b_i) = \sum_{i=1}^{n}a_i + \sum_{i=1}^{n}b_i$$ I have tried in two ways. My first try uses recursive induction: ...
0
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0answers
19 views

What is a purely inseparable extension?

There are many different definitions of purely inseparable extension, and below is what I have chosen for my definition. (Since I don't know what is a standard one, if you know please tell me what ...
3
votes
1answer
63 views

Please check my demonstration of de l'hopital's rule

I have demostrate the de l'hopital theorem but in some steps I'm not 100% sure; The theorem I demostrate is for: $\lim_{x\rightarrow a+} \frac{f'(x)}{g'(x)}=L \implies\lim_{x\rightarrow a+} ...
1
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2answers
29 views

Prove trigonometric identity, hence or otherwise find the general solution

The following question requires one to prove the below trigonometric identity $$\cos 3x = 4\cos ^3 x - 3\cos x$$ Hence, or otherwise, find the general solution of the following equation $$(4\cos ^2 x ...
2
votes
1answer
27 views

Normal convergence: $\sum_{n = 1}^{\infty} \frac{x}{(1 + (n-1)x)(1 + nx)}$

I want to prove that: $$\sum_{n = 1}^{\infty} \frac{x}{(1 + (n-1)x)(1 + nx)}$$ is not normally convergent on $[a, \infty)$ for fixed $a>0$. Let $U_n(x)$ denote the general term. We have: ...
2
votes
1answer
38 views

Is my answer correct? (Devious auction game)

(Taken from here) The question was A man is auctioning a real $20\$$ bill. There are a vast number of bidders. A person may make as many bids as he wants. The starting bid is $5\$$. No $2$ ...
0
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0answers
25 views

Basis of square matrices

Find a basis of the space of complex $n \times n$ matrices, all the elements of which are invertible matrices. I suggest the following: using transvections for $i\neq j$ $T_{i,j}(1) := ...
0
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0answers
23 views

Triangles which are on the same base and in the same parallels equal one another.

I have a small question regarding proposition 37 of the elements of Euclid. http://aleph0.clarku.edu/~djoyce/java/elements/bookI/propI37.html The only problem I got with the proof is the fact that we ...
0
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0answers
25 views

associated prime of a module

Let $f: A\rightarrow B$ be a homomorphism of Noetherian rings, and $M$ a $B$-module. Question: Is $^af(Ass_B(M))=Ass_A(M)$? If $q$ is an associated prime of the $B$-module $M$, $p=^af(q)$, then from ...
1
vote
1answer
29 views

Proving limit of a 2D sequence using $\epsilon-N$ definition

I'm practicing proving a limit of a sequence $a_n:\mathbb{N}\to\mathbb{R}^2$ and I would like to know if I'm doing things correctly. The original limit is ...
0
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0answers
13 views

Straight lines which join the ends of equal and parallel straight lines in the same directions are themselves equal and parallel

I have a small question regarding proposition 33 of the elements of Euclid. http://aleph0.clarku.edu/~djoyce/java/elements/bookI/propI33.html We want to prove that two lines joining equal parallels ...
0
votes
1answer
15 views

If $G$ is a connected graph with $n$ vertices and$ n - 1$ edges then $G$ is a tree, using Induction.

I am still new to proof methods and not sure if this is the correct use of induction. Base case: $n = 1$ has $0$ edges and is a tree. Assume every connected graph with $k$ vertices and $k-1$ edges ...
3
votes
0answers
34 views

$\Box u= | u |^2 u$ global solution in $C^\infty$

Let $u_0, u_1 \in C^\infty( \mathbb{R},\mathbb{R}^3)$. Consider the cubic defocusing NLW $$(\ast)\begin{cases} \Box u= |u|^2 u \\ (u,\partial_t u) \restriction_{t=0} = (u_0,u_1) ,\end{cases}$$ where ...
0
votes
1answer
24 views

Polynomial irreducibiliy with substitution (need evaluation of logic)

One thing I have seen several times when trying to show that a polynomial $p(x)$ is irreducible over a field $F$ is that instead of showing that $p(x)$ is irreducible, I am supposed to show that $p(ax ...
2
votes
2answers
59 views

Proving $0x=0$ in a ring

I am trying to prove the above trivial statement. I am aware of the standard approach of letting $0 = 0 + 0$ and cancelling, but I would like the below statement to be verified/corrected: $1\cdot ...
1
vote
1answer
17 views

Can I assume a condition in the consequent?

Im reading Axler's Linear Algebra Done Right. In an exercise, he ask to prove that $$a\in F,v\in V,av=0 \implies a=0 \lor v=0 $$ where $V$ is a vector space over the field $F$. I've proved it this ...
0
votes
2answers
28 views

If $\forall V\subseteq X$ where $x\in \overline V; f(x) \in \overline{f(V)}$, then $f$ is continous in $x$

Let $f:(X,\tau_X)\to (Y,\tau_Y)$ Prove: If $\forall V\subseteq X$ where $x\in \overline V; f(x) \in \overline{f(V)}$, then $f$ is continous in $x$. Could someone verify the following proof? ...