For questions about the formulation of a proof, not about the mathematics behind it.

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Proving a basis in linear algebra

So at the moment I'm trying to go through proofs and I came across this one: Suppose $ P_n $ is the vector space of all polynomials with degree less than or equal to n. Prove that $ \{1, x − 1, ...
2
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1answer
38 views

Show that for propositional logic $\vdash_i \neg \varphi \Leftrightarrow \vdash_c \neg \varphi$.

As the title says, where $\vdash_i$ is derivations in Intuitionistic logic and $\vdash_c$ is derivations in Classical logic. I am allowed to use a corollary that states that $\vdash_i \varphi ...
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1answer
26 views

How to use monotone convergence theorem to show that $\int \sum |f_n| = \sum \int |f_n|$

In https://math.la.asu.edu/~quigg/teach/courses/473/2009/lectures/11integral.pdf, pg 4 The monotone convergence theorem was used to state $$\int \sum |f_n| = \sum \int |f_n|$$ without proof. Can ...
2
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1answer
24 views

How to show that all sequentially compact spaces are bounded?

I want to show given a sequentially compact subset $A \subseteq M \implies A$ is bounded. I read this Every sequentially compact set is closed and bounded. but the proof is poorly written and jumpy ...
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1answer
27 views

Clean proof for showing $f^{-1}([a,\infty))$ measurable implies $f^{-1}([a,b))$ measurable

I wish to show that for $f:\mathbb{R} \to \mathbb{R}$, $f^{-1}([a,\infty))$ measurable implies $f^{-1}([a,b))$ measurable Looks fairly easy if $f^{-1}([a,b))$ is one piece. Suppose ...
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2answers
43 views

How to prove that the square matrix $A_{n}$ matrix is nilpotent such that $A^{(n-1)}=0$

The matrix A looks like this: $$A=\begin{bmatrix} 0 & 1 & 0 & 0 & .&.&. &0\\ 0 & 0 & 2 & 0 & .&.&. &0\\ 0 & 0 & 0 & 3 ...
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1answer
29 views

Verifying multiplicative inverses of modulo n are the elements that are relatively prime to n

A proposition in my book states: $(\mathbb{Z}/n\mathbb{Z})^{\times} = \{a \in \mathbb{Z}/n\mathbb{Z}~|(a,n) = 1\}$ which I want to prove. I start by defining $a$ in terms of prime factors $$a = ...
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3answers
45 views

Proof help: Prove that $x^2+y^2+z^2 \geq xy+xz+yz$ [duplicate]

$x^2+y^2+z^2 \geq xy+xz+yz $ for all real numbers, x, y, and z. I'm not very good with working inequality proofs. Can someone help me prove this? The technique doesn't really matter.
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0answers
18 views

Existence of convergent sequence in a closed set?

Is it true that if $C$ is a closed set, and $x \in {C}$ then there exist a sequence $\{x_i\}$ which tends to $x$? I'm not sure about the correctness of this statement, and whether it is true for ...
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3answers
45 views

Prove that graph with odd number of odd degree vertices does not exist

I need to prove that it is impossible to have a graph in which there are an odd number of odd degree vertices. What is the easiest way to formally prove this? I feel that I can prove it just by ...
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2answers
37 views

Prove that if A and B are sets such that $A \cup B \neq \emptyset$, then $A \neq \emptyset$ or $B \neq \emptyset$

Prove that if A and B are sets such that $A \cup B \neq \emptyset$, then $A \neq \emptyset$ or $B \neq \emptyset$. It was suggested to me that the easiest way to approach this was with a proof by ...
2
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2answers
28 views

Proof outer measure satisfies monotonicity: $A \subseteq B \implies m^*(A) \leq m^*(B)$

Theorem: $$A \subseteq B \implies m^*(A) \leq m^*(B)$$ Proof Attempt: By definition, $m^*(B) = \inf\{\sum\limits_{k=1}^\infty |J_k||\{J_k\} \text{ is a cover of B }\}$, $m^*(A) = ...
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2answers
30 views

Moments of a continuous random variable (Exercise 4.3.3 from Grimmett and Stirzaker)

Let $X$ be a non-negative random variable with density function $f$. Show that $$ E(X^r) = \int_0^\infty r x^{r-1} P(X > r)\,dx. $$ I tried using integration by parts to obtain \begin{align} ...
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0answers
17 views

Suppose G and G' are isomorphic and G has m vertices of degree k. Prove that G' has m vertices of degree k

Suppose G and G' are isomorphic and G has m vertices of degree k. Prove that G' has m vertices of degree k. I don't know how to start this one
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1answer
27 views

Need help understanding algebra steps taken in proof of why an even minus an odd is odd

I don't understand the algebra used in the below example proof from my textbook. Where does the + 1 come from? Is it okay to just add 1 anywhere you want? Or is there some rule here or reason you ...
5
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1answer
89 views

Is my proof correct? If $f$ has a finite number of discontinuities on $[a, b]$, then it is integrable on $[a, b]$

Question: Suppose a function $f(x)$ over the interval $[a, b]$ is bounded and has only a finite number of discontinuous points on $[a, b]$. I intend to prove that it must be integrable on $[a, b]$. ...
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1answer
27 views

Proof of Interceting Lines

I have this practice problem from a final exam study guide. Let $f,g$ be continuous on $[a,b]$ and $f(a)>g(a)$ but $g(b)>f(b)$. Prove that $\exists c \in [a,b]$ such that $f(c)=g(c)$. My ...
2
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0answers
35 views

Simple Vacuous Proof, Correct Approach?

I am doing some practice exercises as I am starting out on proofs but I noticed that though I am getting the correct approach between vacuous and trivial proofs, I am not doing it in the same format ...
1
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1answer
16 views

How does one consider what a graph looks like in a mathematical proof

Mostly I am wondering for example what it would be like to prove that a linear graph (negative slope) shifted right would look the same as one shifted up. Can you consider how a graph looks when ...
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0answers
41 views

When is it appropriate to write “Then it follows”

I am reading a proof, and before the proof fully finishes, the author writes "Then it follows [the statement we are trying to prove] is true" I have been spending the last three hours justifying the ...
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1answer
29 views

How to verify this relationship between area under the graph and the preimage?

Define $h : \mathbb{R} \to [0, \infty)$, Let $H = \{(x,y)| 0 \leq y \leq h(x)\}$ be the area under the graph (including the boundary) I wish to show the following is true: $$H = ...
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0answers
38 views

Why is this proof that a circular cone is not a surface not rigorous?

In example $4.1.5$, page $73$ of Pressley's Elementary Differential Geometry, a "heuristic" argument is given to prove that the circular cone with vertex the origin and angle $\pi/4$, is not a ...
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0answers
57 views

I did not understand one thing in the proof of substitution lemma?

The substitution lemma in lambda-calculus is proved by the following way, but I just did not understand the application of induction hypothesis in it. The lemma as shown below, where x and y are ...
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0answers
25 views

Real Analysis Theorems - Help with this proof

I am having problems trying to prove the following. Given a set A = {(n,1) / n Natural and n < 6} and given F: R*R -> R, a function that belongs to C1 class and F(P) = 0 for any P inside A. I need ...
0
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1answer
15 views

Prove that multiplication by an integer $a$ that is relatively prime to $n$ defines a bijection from $\mathbb{Z}_n-\{0\}$ to itself

If gcd$(a,n)=1$, then multiplication by $a$ defines a bijection from $\mathbb{Z}_n-\{0\}$ to itself. My working: If $n=p$ a prime, then we can use the Fermat's Little Theorem. If $n$ is not prime in ...
0
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1answer
11 views

Show that monotonicity implies positive definiteness of the Jacobian

Given $f: \mathbb{R}^n \to \mathbb{R}^n$, $f$ differentiable, $x,y, p \in \mathbb{R}^n$, show that $(x-y)^T(f(x) - f(y)) \geq 0 \Leftrightarrow p^TDf(x)p \geq 0, \forall p \in \mathbb{R}^n$ This ...
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3answers
42 views

How to prove that $\sum_{k=0}^{n-1}\cos\left(\frac{2\pi k}{n}+\phi\right)=0$ [duplicate]

Prove that $\displaystyle\sum_{k=0}^{n-1}\cos\left(\frac{2\pi k}{n}+\phi\right)=0$ for $n\in\mathbb{N},n>1$ I'm thinking at a demonstration by induction, as base case $n=2$ ...
0
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1answer
25 views

Proof on showing the integral of f(x)=0

I am having difficulties showing that if $f$ is continuous such that $f(x)<0$ for every $x \in (a,b)$ then $\int_a^bf(x)\;dx <0$ I am given the theorem that if $f$ is continuous such that ...
2
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2answers
21 views

Why Is This Squared Modulo Prime Number Equation True?

I recently was trying to figure out if there was an simple way to tell how many unique outcomes can be produced from the following equation: $k^2 \mod m$ where $m$ is some odd prime number and $k$ ...
2
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3answers
102 views

Prove that there is no largest irrational number

I have to prove that there is no largest irrational number from the result of the a previous proof: Prove that if $x$ is rational and $y$ is irrational then, $x+y$ is irrational. I was able to prove ...
21
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5answers
1k views

How many faces of a solid can one “see”?

What is the maximum number of faces of totally convex solid that one can "see" from a point? ...and, more importantly, how can I ask this question better? (I'm a college student with little ...
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2answers
24 views

How to prove this trig identity turning $\tan$ into $\cot$?

$$\frac{\tan u - \tan v}{1 + \tan u \cdot \tan v} = \frac{\cot v - \cot u}{\cot u \cdot \cot v+1}$$ I've been trying to prove this for a while, no luck (I do know it's true). I've attempted to turn ...
1
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1answer
13 views

How to prove columns of matrix $A$ are linearly independent $\implies$ $C$ must be invertible for the following condition?

Suppose $A=BC$, where $B$ is an $m\times n$ matrix and $C$ is an $n\times n$ matrix .How to prove columns of $A$ are linearly independent $\implies$ $C$ must be invertible? In my opinion, I feel like ...
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0answers
13 views

Help with a possible Cauchy Proof?

Hello Mathematics community! I have this problem I've been stuck on for a while: Suppose $a_n$ is a sequence, and {$L_1, L_2,L_3,...$} are partial limits of $a_n$. Assume that $L_n$-> $L$. Prove that ...
1
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1answer
19 views

Prove that $\displaystyle\int_{x=-1}^{1}P_L(x)P_{L-1}\acute (x)\,\mathrm{d}x=\int_{x=-1}^{1}P_L\acute(x)P_{L+1} (x)\,\mathrm{d}x=0$

A question (Problem $7.4$) in my textbook (Mathematical Methods in the Physical Sciences - 3rd Edition by Mary L. Boas P578) asks me to Use $$\int_{x=-1}^{1}(P_L(x)\cdot\text{any polynomial of ...
3
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1answer
66 views

Proving $\lim_{x\to 1}\frac{2+4x}{3}=2$ using the $\epsilon$ -$\delta$ definition of a limit

I'm looking for a verification of my $\epsilon-\delta$ proof of a limit example, if my proof is not completely mathematically rigorous, please tear it apart. Required to Prove: $$\lim_{x \to 1} \ ...
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3answers
89 views

Prove that $(x^2+1)\mathbb Z[x]$ is a prime ideal of $\mathbb Z[x]$, but not maximal

Prove that $(x^2+1)\mathbb Z[x]$ is a prime ideal of $\mathbb Z[x]$, but not maximal. I'm supposed to show this for my homework. My first thought is to show that $\mathbb Z[x]/(x^2+1)\mathbb ...
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2answers
29 views

Proofs Using Tautologies

Let's say I want to formally prove a statement of the form $$p \implies q$$ So I do a bit of work,some re-arranging and eventually I arrive at a statement of the form $$p \implies p$$ which is a ...
1
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1answer
16 views

Proof Explaination: Show the set of measurable sets is closed under finite union

I have a proof of the above claim but I think there are some mistakes, I have highlighted them I hope someone could help figure out exactly what is wrong. Given $\omega$ an outer measure on set ...
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3answers
34 views

Verification of a proof in Measure Theory

Let $m$ be the Lebesgue measure on $\Bbb R$ and $f:\Bbb R\to [0,\infty)$ be a Lebesgue integrable function. Show that $\exists $ a measurable set $E\subset [0,\infty)$ such that $m(E)\neq m(f^{-1}(E)$ ...
0
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1answer
22 views

Polynomials and Lipschitz function

Let $f(x) = x^4 + 11x^2 + 9x -5$ and let $M > 0$. Show that f is a Lipschitz function on the interval $[-M, M]$ I honestly cannot figure out how to start this proof. Nothing similiar is in the ...
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0answers
9 views

Nonuniform Continuity Criteria with cos(1/x)

Use the nonuniform continuity criteron to show that $f(x) = cos\left(\frac{1}{x}\right)$ is not uniformly continuous on $(0, \infty)$ My proof: Let $(x_n)$ be defined by $x_n = \frac{1}{2n\pi}$ ...
1
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2answers
19 views

If $Mod(T_1 \cup T_2) = \emptyset$ then for some $\sigma$, $T_1 \vDash \sigma$ and $T_2 \vDash \neg \sigma$

Problem description: if $T_1$ and $T_2$ are theories such that $Mod(T_1 \cup T_2) = \emptyset$, then there is a $\sigma$ such that $T_1 \vDash \sigma$ and $T_2 \vDash \neg \sigma$. I don’t ...
1
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2answers
59 views

Is there a symbol for always less than (or just always?)

For e.g, the quotient of $\frac{1}{n}$, $q$, where $n \gt 1$, $q$ will always be less than $1$. $$\frac qn\le n$$ etc. I can't really write $\frac {q}{n} < n$, because whilst true, it doesn't ...
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3answers
96 views

Prove that $3^x$ divides $(3x)!\,\,\,\, \forall x \ge 1$ [closed]

Prove that $3^x$ divides $(3x)!\,\,\,\, \forall x \ge 1$ For example, $3$ divides $6 = 3!$
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2answers
41 views

Show that well-ordering is not a first-order property.

Problem description: Show that well-ordering is not a first-order notion. Suppose that $\Gamma$ axiomatizes the class of well-orderings. Add countably many constants $c_i$ and show that $\Gamma \cup ...
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3answers
40 views

Does proof proceed from left to right?

Very simple question: if we're asked to prove that $a=b$, do we start with $a$ and then find $b$ from $a$? Does going the other way around count as a formal proof? The exercise in question is this: ...
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3answers
32 views

How to prove that the g.c.d is equal to the prime factorization raised to the minimum of two powers

for the prime factorization of $a$ and $b$ as $$a = p_1^{\alpha_1}p_2^{\alpha_2}\cdots{p_t}^{\alpha_t}$$ and $$b = p_1^{\beta_1} p_2^{\beta_2} \cdots p_s^{\beta_s}.$$ I want to prove that $d = (a,b)$ ...
1
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1answer
38 views

show that $3^{1974} + 5^{1974} \equiv 0 \bmod 13$

show that $3^{1974} + 5^{1974} \equiv 0 \bmod 13$ My attempt with this question was to use Fermate Little's THM. But I do not understand how to properly use it for this question. Can some one show me ...
0
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1answer
18 views

Proof of equivalency in disjoint sets.

Prove, If A, B, C, and D are sets with |A|=|B| and |C|=|D| and if A and C are disjoint and B and D are disjoint, then |A ∪ C|= |B ∪ D|. Would I start this proof using the definition of disjoint ...