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

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1
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0answers
13 views

All closed rational rays measurable implies $f$ measurable

Is the following proof correct? Let $f: X \to \mathbb{R}$ where $X$ is a measurable space. Suppose $\{x: f(x) \geq r\}$ is measurable for each $r \in \mathbb{Q}$. Then, $f$ is measurable. Proof: ...
1
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1answer
12 views

Show $R \setminus S$ is a union of prime ideals

I'm stuck on the following question: Let $R$ be a commutative ring with $1$, and $S \subseteq R$ a saturated multiplicative set (that is, $1 \in S$ and $x, y \in S$ if and only if $xy \in S$). ...
0
votes
1answer
13 views

If $x,y,a_1,\ldots,a_n\in\mathbb Z$ and $a_1,\ldots,a_n\in[x,y],$ then $a_1=x,a_2=x+1,\ldots,a_n=y$ (proof verification)

I recently solved a problem in which I used the following fact. If there are exactly $n$ integers in the interval $[x,y]$ ($x,y\in\mathbb Z$) and $a_1,a_2,\ldots,a_n$ are integers of that interval ...
1
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0answers
19 views

Alternative proof of the Riemann Sum Theorem using Mean Value Theorem for Integrals.

I've been reviewing proofs for a couple of calculus theorems and as I was trying to recall the proof of the Riemann Sum Theorem which uses Lower Sums and Upper Sums I came up with an idea to prove it ...
0
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2answers
36 views

Let $X$ and $Y$ be finite sets. Then $X \cup Y$ is finite and $| X \cup Y| \leq |X| + |Y|$.

Let $X$ and $Y$ be finite sets. Let us assume that they are distinct at least, for otherwise $X \cup Y = X$ and $X$ is finite. Also let us assume that $X$ has cardinality $n$ and $Y$ has cardinality ...
2
votes
4answers
36 views

Prove $\log(x) < n(x)^{1/n}$, for all positive integer values of $n$, and $x > 0$

Given that $$lg(u) < u$$ is always true, how do we use that to prove that $$lg(x) < n(x)^\frac 1n$$ These are the steps that I have taken so far: $$1: lg(x) < n(x)^\frac 1n$$ $$2: \frac ...
1
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0answers
16 views

On n! divided by a product of primes and related questions

We have the following Definition 1. For integers $n\geq 1$ we define $$f(n) = \begin{cases} 1, & \text{if $n=1$} \\[2ex] \frac{n!}{\prod_{p\leq n}p}, & \text{if $n>1$} ...
5
votes
1answer
57 views

Proof of Vandermonde's Identity using a “different approach” using complex integration

Hi I'd like to know if the following proof of Vandermonde's Identity is correct (is really easy): Let $m,n,r$ be natural numbers such that $r\le \min \{m,n\}$. The Vandermonde's Identity gives us ...
-1
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1answer
41 views

I want to appeal this problem from an exam in Linear Algebra I, do you think its appealable? [on hold]

I have the follow question : Let $U_1, U_2, W$ are linear spans of linear space $V$ while V is finite. Proof: If $$U_2 \cap W \neq \{0\}$$ $$U_1\cap W\neq \{0\}$$ $$U_1 \cap U_2=\{0\}$$ Then $dimW ...
2
votes
1answer
25 views

Understanding pasting lemma proof

Let $A$ and $B$ be both open or closed subsets of a topological space $X$ such that $A \cup B = X$. Let $f: A \to Y$ and $g: B \to Y$ be continuous such that $f = g$ for all $x \in A \cap B$. Prove ...
1
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2answers
24 views

Prove: For every $\epsilon > 0$, there exists a $\delta >0$ such that $1 - \delta < x < 1 + \delta$ implies that $2 - \epsilon < 7 -5x< 2 + \epsilon$

So far this is what I have Let $$ \delta = \frac{\epsilon}{5} $$ So, if we start with $1 - \delta < x < 1 + \delta$ \begin{align} &\Rightarrow -5 + 5\delta < -5x ...
7
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1answer
64 views

Functional equation: Show $0\le f(n+1)-f(n)\le 1$ and find all $n$ such that $f(n)=1025$.

The function $f:\mathbb{N}\to \mathbb{R}$ satisfies all of $$\begin{align}f(1)&=1, \\ f(2)&=2,\\ f(n + 2) &= f(n + 2 − f(n + 1)) + f(n + 1 − f(n)) \tag{1} \end{align}$$ Show ...
2
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0answers
33 views

For any Subspace $A$ of a Path-Connected Space $X$, we have $H_0(X, A)=0$.

I recently learnt about relative homologies and am wondering if the following is true: Statement: Let $X$ be a path-connected topological space and $A$ be a non-empty subspace of $X$. Then $H_0(X, ...
3
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0answers
21 views

Help in step of the proof of Burnside's $p^aq^b$ theorem (Doerk-Hawkes book)

I'm reading the proof of the $p^aq^b$ Burnside's theorem from the book Finite soluble groups by Doerk and Hawkes. The fifth step of the proof says 2.5. Let $M$ and $H$ be maximal subgroups of $G$ ...
5
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0answers
38 views

For finitely generated free abelian groups $A,B$ if there is an onto homomorphism $A \to B$, then $\operatorname{rank}(A) \geq \operatorname{rank}(B)$

$\newcommand{\rank}{\operatorname{rank}}$For two finitely generated, free abelian groups $A,B$ prove that if there is an onto homomorphism $A \rightarrow B$, then $\rank(A) \geq \rank(B)$ Assume that ...
1
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1answer
18 views

Cyclic Groups - $a^k = e \text{ iff } n|k$

I saw this proof in the book on Abstract Algebra. Here is part of it: Let $G$ be a cyclic group of order $n$ and $a$ is the generator of $G$. Then $a^k = e \iff n|k$ Proof: Suppose $a^k=e$. By the ...
1
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1answer
14 views

Proof check on $I (\lim \sup E_n(w)) = \lim \sup I_{E_n}(w)$, where $I$ is the indicator function.

I proceed by cases: $ \lim \sup I_{E_n}(w) := \lim_m \downarrow \{ \sup_{n>m} I_{E_n}(w) \}$ this is $0$ only if $w \not \in E_n \forall n > m$. At the right of the equality we have ...
1
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1answer
30 views

Validity of my proof by contradiction of converse of Pythagorean Theorem.

So in, $\triangle ABC$ it is given that $AB^2=AC^2+BC^2$. Let us assume that $\angle C\neq{90}^{\circ}$. And let us make a perpendicular $AD$ to $BC$. Now, by the Pythagorean Theorem, in $\triangle ...
2
votes
1answer
29 views

Showing that $\mathbb{R}$ is a metric space with the metric $d(x,y)=|x-y|$

I want to show that $\mathbb{R}$ is a metric space with the metric $d(x,y)=|x-y|$. So three properties of the metric space $d(x,y)$ in general needs to be satisfied. My work: Let $x,y \in ...
1
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1answer
19 views

Show that if there is a function $g:Y\to X$ such that $g\circ f$ is the identity function on $X$, then $f$ is one to one.

Let $X,Y$ be nonempty sets and $f:X\to Y$ be a function. Show that if there is a function $g:Y\to X$ such that $g\circ f$ is the identity function on $X$, then $f$ is one to one. My approach is by ...
0
votes
0answers
23 views

Topology generated by basis equals intersection of all topologies that contain A

Here is my proof I was wondering any critiques to my proof. If A is a basis;The topology generated by A equals collections of all unions of elements of A that is $\tau = \bigcup_{i \in I: B_i \in ...
3
votes
0answers
55 views

A quick Galois Theory question

Let $\mathbb{F}_q$ be a finite field of order $q$ where $q$ is a prime power. For any $d \in \mathbb{N},$ we have an inclusion $\mathbb{F}_{q^d} \subseteq \overline{\mathbb{F}}_q.$ Both ...
1
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2answers
15 views

Partitioning a number as a sum of $k$ non-zero numbers, but order does not matter

I would like some confirmation regarding my logic here, which I feel is 'suspiciously straightforward'. Say I wish to express a number as the sum of $10$ non-zero numbers, where order does not ...
0
votes
1answer
16 views

Normal Matrix with Real Eigenvalues is Hermitian

Let $A$ be a normal matrix. Then I want to show that, if $A$ has real eigenvalues, $A$ is Hermitian. (Notation: * denotes the complex conjugate, T denotes the transpose, and $\dagger$ denotes the ...
2
votes
3answers
93 views

Proving $\sin^2(x) + \cos^2(x) =1$ using calculus

Ok so the book in which I found this doesn't say mention the trigonometric functions by name but the question is: Let $s(x)$ and $c(x)$ be functions satisfying $s'(x)=c(x)$ and $c'(x)= -s(x)$ for ...
0
votes
1answer
16 views

Is non-paralellism transitive?

I've been trying to check if non-paralellism is transitive. At the moment, I know it's symmetric. But I have no idea on how to prove that it's transitive. I did the following: $$(a \not\parallel b) ...
1
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1answer
38 views

Exercise from (Baby) Rudin (Chapter 3, exercise 13): is this proof correct? Is it well-written?

The problem is the following: Prove that the Cauchy product of two absolutely convergent series converges absolutely. Here is my attempt: Let $s_n=\sum^n_{k=0}a_k$ and $t_n=\sum^n_{k=0}b_k$ be ...
2
votes
1answer
35 views

Proving that limit of a sequence is 0 from definitions.

I had this question in a test: Use the definition of limit in order to prove that if $\{a_n\}$ (n goes from 1 to infinity) is a sequence of real numbers such that $\lim_{n\rightarrow \infty} a_n^2 ...
1
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0answers
22 views

The restriction of a discontinuous linear functional to any open set is surjective.

Problem. Let $X$ be a topological vector space and $f:X\to\mathbb{K}$ a linear mapping. Prove that if $f$ is discontinuous, then $f(A)=\mathbb{K}$ for all nonempty open set $A\subset X$. I'd like ...
2
votes
1answer
17 views

If $N$ is nilpotent of index $n\geq 2$ but $N^{n-1}\neq 0$ then there's no $A$ such that $A^2=N$

Let $N\in M_{n\times n}^{\mathbb{C}}$ a nilpotent matrix of index $n\geq 2$. Prove: if $N^{n-1}\neq 0$ then there does not exist a matrix $A\in M_{n\times n}^{\mathbb{C}}$ such that $A^2=N$. My ...
1
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1answer
73 views

Is this a valid proof of this math challenge problem?

From a fixed point P not in a given plane, three mutually perpendicular line segments are drawn terminating in the plane. Let a, b, c denote the lengths of the three segments. Show that ...
0
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0answers
46 views

Differentiability of parameter-dependent integrals when derivative exists only almost everywhere

This unanswered question asked in 2013 Differentiation under the Integral Sign (let's call this Q-zero) seems to be taken from this (or pdf ver.). The result on differentiation under the integral ...
0
votes
1answer
21 views

Find a criterion for divisibility

Find a criterion such that $\displaystyle\sum_{i=1}^ni$ divides $\displaystyle\prod_{i=1}^ni^2$ for $n\in\mathbb N$. What I have done so far, $\displaystyle\sum_{i=1}^ni=\frac{n(n+1)}{2}$ and ...
2
votes
2answers
145 views

What makes it legitimate to multiply both sides?

Having the proof of the cancelation law for multiplication: $$cb=ab$$ $$(cb)b^{-1}=(ab)b^{-1}\tag{Inverse}$$ $$cbb^{-1}=abb^{-1}\tag{Associativity}$$ $$c\cdot 1=a\cdot 1\tag{Indentity}$$ $$c=a$$ ...
3
votes
5answers
109 views

Proof check: $(4n)!$ is divisible by $2^{3n}3^{n}$

Question: Show that $(4n)!$ is a multiple of $2^{3n}3^{n}$ for all $n$. Proof: It's easy (involves kinda messy calculation tho) to show by induction that $(4n)!$ is a multiple of $2^{3n}$. Now, since ...
0
votes
2answers
36 views

Weak convergency vs strong convergency in Hilbert space

Let $\mathcal{H}$ be an Hilbert space and let $(x_n)_n \subset \mathcal{H}$ be a sequence s.t. $$ x_n \rightharpoonup x ~~~,~~~ \| x_n \| \to \|x\| $$ We want to show that $ x_n \to x $. Now, I ...
0
votes
1answer
31 views

Lower bound for $\Pi(n)$ - viability of probabilistic theory

Can somebody check the validity of my arguments below, and tell me why its wrong or right? Consider the sequence of non-negative integers. Let $a_0=0, a_1=1, ..., a_i=i,...$ Divisiblilty of $a_i$ ...
4
votes
0answers
18 views

Iterated circumcenters - proving collinearity and establishing distance ratios

Let $P_0, P_1, P_2$ be three points on the circumference of a circle with radius $1$, where $P_1P_2 = t < 2$. For each $i \ge 3$, define $P_i$ to be the centre of the circumcircle of $\triangle ...
1
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1answer
33 views

Find all primes $p$ for which $x^2+2x+4\equiv 0 \pmod p$ is solvable. Am I correct?

Getting ready for an exam, I would like to focus on the correctness of my solution, final results and assumptions, and would appreciate any comment regarding it or even additional ...
1
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2answers
29 views

Proving that $a^{b}$ is rational (Elementary number theorey) [duplicate]

Prove that there exist irrational numbers $a$ and $b$ such that $a^{b}$ is rational. What i tried Prove by contradiction I assume the statement For all rational numbers $a$ and $b$ such that ...
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0answers
42 views

Problem 14 from Baby Rudin chapter 3

Let $\{s_n\}$ is a complex sequence, define its arithmetic means $\sigma_n$ by $$\sigma_n=\dfrac{s_0+s_1+\dots+s_n}{n+1},\quad a_n=s_n-s_{n-1} \quad\text{for} \quad n\geqslant 1$$ Assume ...
1
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1answer
36 views

$\gamma=(\psi \implies \phi)$ is a tautology $\equiv \psi$ is a contradiction or $\phi$ is a tautology.

Prove that: If $\psi,\phi$ are formulas such that $\text{VAR$(\psi)$} \cap\text{VAR$(\phi)$}=\emptyset$. Then $\gamma=(\psi \implies \phi)$ is a tautology $\equiv \psi$ is a ...
0
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1answer
13 views

Show that a line is tangent to a circle in the extended complex plane.

The straight line $l$ in the extended-complex plane pasess through $2+i,2+2i$.The circle $C$ centered at $-1-2i$ with radius $3$. First, I find the parametrization of the straight line which is $$z = ...
3
votes
4answers
81 views

Proof of $(A\cup B)-(A\cap B)=(A-B)\cup(B-A)$

I was trying to prove $(A\cup B)-(A\cap B)=(A-B)\cup(B-A)$ and came across issues in translating (pertaining to what I did with $\emptyset$) and got through the proof but was doubting its accuracy so ...
6
votes
3answers
142 views

Prove that $\det(I-CD)=\det(I-DC) $

Let $C$ and $D$ be matrices such that $DC$ and $CD$ are square matrices of the same dimension. How can one prove that $\det(I-CD)=\det(I-DC)$? This is my approach to the question. I am not sure ...
0
votes
0answers
20 views

Generalized Associative Property (Proof Verification)

I am really confused about Associative property and Generalized associative property. I am not sure of my proof, and I have a feeling that it is not correct. Would be happy if someone can tell me what ...
1
vote
2answers
39 views

Formal power series over a regular ring is regular

I'm trying to prove that if $A$ is a regular ring then so is $A[[X]]$. The only proof I found of this statement is in Commutative Ring Theory by Matsumura, but it seems a bit over my knowledge so I'd ...
3
votes
2answers
31 views

Proving a function $F$ is surjective if and only if $f$ is injective

Problem: Let $X$ and $Y$ be non-empty sets and let $f: X \rightarrow Y$ be a function. Then we can define $F: P(Y) \rightarrow P(X)$ by \begin{align*} F(B) = f^{-1}(B) \qquad \text{for all} \ B \in ...
5
votes
0answers
49 views

A natural number has exactly 10 divisors including 1 and itself.How many distinct prime factors can this natural number have?

A natural number has exactly 10 divisors including 1 and itself.How many distinct prime factors can this natural number have? options given: (a) either $1$ or $2$ (b)$1$ or $3$ (c)either $2$ or ...
0
votes
1answer
37 views

Check whether this is indeed a counterexample

Let $A,B \subset \mathbb{R}$; let $Q := A \times B$; and let $f: Q \to \mathbb{R}$ be bounded. The problem is to give a counterexample to the proposition that if the Riemann integral $\int_{Q}f$ ...