This tag is for readers who ask for explanation and clarification of some steps of a particular proof.

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2
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2answers
67 views

$\pi(x)$ Proof Clarification

In a proof from a number theory book that $${\pi(x) \over x}\le {2k \over x} + {\phi(k) \over k}$$ Where $x=kl+r$ with $0 \le r\lt k $ It is stated that $$\pi(x) \le k+(l-1)\phi(k) + r \le 2k+{x\over ...
1
vote
1answer
37 views

Question about functions that are not uniformly continuous.

I am not asking for any particular question today rather I need some help to understand the concept of a function that is not uniformly continuous. Ok, so If I understand this correct, to prove a ...
3
votes
1answer
18 views

Understanding a matrix bound/inequality

I came across the following statements; For a positive matrix $A$, that is bounded $0 \leq A \leq I,$ where $I$ is the identity matrix and a statement like $Y \leq X$ means that $X-Y$ is positive ...
2
votes
0answers
45 views

Derivation of Backward Kolmogorov Equation

I'm following Kallianpur-Gopinath's textbook "Stochastic analysis and diffusion processes" to study Kolmogorov equations and I got stuck in a step of the derivation of the backward equation. In ...
0
votes
1answer
15 views

Understanding part of a proof, to show commutatitivy of geometric mean for matrices

Ando defined a matrix geometric mean, for two positive $n \times n$ matrices, as follows: $$G(A,B) = B^{1/2}(B^{-1/2}AB^{-1/2})^{1/2}B^{1/2}$$ where all square roots are positive square roots. This ...
2
votes
2answers
81 views

Polynomial in several variables over $GF(2)$

Can anyone please explain how this Lemma has been proved? Lemma: Let $f$ be a nonzero polynomial in variables $x_1,\ldots,x_n$ over $GF(2)$, and let $d$ be the maximum degree of $f$ with respect to ...
7
votes
1answer
119 views

Mean value theorem for random variables (inside an expectation value)

In a proof I am trying to understand a mean value theorem for random variables is used. It is stated that $$E[f(X+Y)]=E[f(X)+E[f^\prime(X+\theta Y)]Y]$$ for real valued random variables $X$ and $Y$ ...
3
votes
2answers
76 views

Theorem 7.2 in General Topology by S. Willard

Theorem 7.2 If $X$ and $Y$ are topological spaces and $f:X \to Y$ , then the following are all equivalent :- I) $f$ is continuous. II) for each E $\subset X$ , $f(\bar E) \subset ...
0
votes
1answer
39 views

Looking for explanation of Banach-Tarski Proof, preferably by visual methods “Video, Pictures, Diagrams…”

could someone please explain the four steps of Banach-Tarski? 1- Find a paradoxical decomposition of the free group in two generators. 2- Find a group of rotations in 3-d space isomorphic to the free ...
-1
votes
2answers
18 views

On Closure of Product subset of $\Bbb R×\Bbb R$

Suppose that $\Bbb R×\Bbb R$ has the standard topology. If $S=\left\{(t,\sin{\frac{1}{t}})\mid t\in R\text{ and }t\gt 0\right\}$. Show that $(0,0)$ $\in \overline{S} $
0
votes
0answers
14 views

Queries on proof of Plancharel's theorem

I have been provided with a proof for plancharel's theorem: $$\int^\infty_{-\infty} |f(t)|^2 \ {\rm d}t = \int^\infty_{-\infty} |\hat{f}(\omega)|^2 \ {\rm d}\omega.$$ proof. Let $$f(t) = ...
1
vote
1answer
63 views

Question 7F from general topology by Stephen willard?

Can someone help me with 7F from Willard? In part two : $\mathbf{7}$F. Functions to and from the plane. The facts presented here for the plane will be proved in more generality for ...
3
votes
2answers
33 views

What is the best way to explain setting a restriction on $\delta$ in $\epsilon$-$\delta$ proofs?

I'm trying to prepare a somewhat informal lesson striving to provide an intuitive understanding of why for some limit proofs, we have to set an upper bound on $\delta$. For example, here's part of ...
1
vote
1answer
46 views

PDE - Don't understand teacher's solution

I'm reading a solution to a problem in PDE class that the teacher gave, and I don't fully understand his solution. The problem is $\frac{dx}{1+\sqrt{z-x-y}}=dy=\frac{dz}{2}$ and what we want is to ...
2
votes
0answers
30 views

$R$ commutative ring with unity. Prove if $R/M$ is a field $\implies M$ is a max ideal.

$R$ commutative ring with unity. Prove if $R/M$ is a field $\implies M$ is a max ideal. Let us propose the opposite: that M isn't the max ideal. So $$I \triangleleft R, M\nsubseteq I$$ (How this is ...
0
votes
1answer
50 views

Barendregt's Substitution Lemma (lambda calculus)

I am struggling to put words on an idea used in Barendregt's Substitution Lemma's proof. (available here) The lemma states that: If x≠y and x not free in L and M, L are $\lambda$-terms: then ...
1
vote
1answer
28 views

Mittag-Leffler Proof. Rudin notation question and some basic real analysis/topology

Proof is below What is meant by $\sum_{\alpha \in A_n}$? I thought $P_\alpha$ is a sum already. What is this open set he is talking about? Because $A_n$ isn't open. Oh he means ...
2
votes
2answers
35 views

What is meant by “regularized rank-1 approximation”?

Suppose we have the following formula: $$\forall i: \{d_i, x_{[i]}^T\}={\mathrm{argmin}_{d,z}} 1/2 ||E_i-dz^T||_F^2+ \lambda|z||_1 $$ I really don't get how the following sentence is concluded and ...
0
votes
2answers
18 views

What is $P$ and $X$ is supposed to be in this analysis question?

Source page 626. Can someone explain what is $\| P\| $ mean? Is that partition or what? Also why bother with $\epsilon/2$ if the giant expression in the middle proves the lemma. Finally, ...
0
votes
0answers
25 views

Existence of nice exhaustion - Rudin.

This is taken from Rudin's Complex Analysis/Real Analysis Can someone tell me why $K_n \subset \Omega$? I agree it is compact, but why does it follow that it is a subset of $\Omega$? WLOG, I ...
1
vote
1answer
75 views

On the associative property of a binary operation of the fundamental group.

I was reading about the proof of associativity property of the operation on the fundamental group here. The book gives the following diagram then it says the reader should supply the elementary ...
0
votes
0answers
42 views

Minor issue about the proof of the Cauchy Convergence Criterion on Understanding Analysis (Abbott)

I don't understand one small part of the proof of the following statement: If a sequence is a Cauchy sequence, then it converges. PROOF: Let $(x_n)$ be a a Cauchy sequence. Then it is bounded ...
0
votes
1answer
22 views

Discrete Math - Relation among the relations

I need help understanding an assignment involving sets. We're given a few sets, $A=\{1,2\}$, $B=\{1,2,3\}$, $C=\{1,2,3,4\}$, $P=\{(1,1),(2,3),(3,2)\}$, $Q=\{(1,1),(2,2),(3,2)\}$, ...
0
votes
0answers
23 views

blocking set of projective planes

Have a quick question, considering the Desarguesian projective plane of order q, what is an upper bound of the minimal blocking set? Wikipedia says the size of the blocking set is bounded below by ...
1
vote
1answer
40 views

Explanation of a proof about continuity in Spivak's Calculus

I can't see the "it follows that" part of the following proof in Spivak's Calculus book: Given that $f$ is continous at $b$, there is a $\delta>0$ such that; if $|x-b|<\delta$ then ...
1
vote
2answers
64 views

Regarding the proof of $\int_0^xf(u)(x-u)du$

This question has already been asked: For continuous function $f$, prove: $\int_{0}^{x} \; \left[\int_{0}^{t}f(u) \;du \right] \;dt=\int_{0}^{x} f(u)(x-u)du$ but I really don´t understand the part ...
0
votes
2answers
59 views

$\det(aI-T) = 0$ implies $a$ is an eigenvalue of $T$

In Hoffman and Kunze's linear algebra, right after the definition of an eigenvalue for an endomorphism $T: V \to V$ (i.e. $a\in F$ is an eigenvalue if there exists $\alpha\in V$, $\alpha \neq 0$, such ...
0
votes
1answer
30 views

Not understanding a line in a proof concerning Monomorphism and injectivity

In the proof that "in the category Div of divisible (abelian) groups and group homomorphisms between them there are monomorphisms that are not injective" given in Wikipedia ...
3
votes
1answer
75 views

No cycle containing edges $e$ and $g$ implies there is a vertex $u$ so that every path sharing one end with $e$ and another with $g$ contains $u$

There is a proof in my textbook for the following claim, which doesn't make a whole lot of sense to me. My annotations are in bold. Could someone perhaps elaborate on what's going on? Claim. If ...
0
votes
0answers
35 views

Theorem 2 of Section 5.4 in Fulton's Algebraic Curves

The Theorem and its proof are: To understand the proof we also need: Finally, Corollary 1 for Bezout's Theorem says that $\sum_{P \in F \cap G} m_P(F) m_P(G) \le deg(F) deg(G)$ for any curves ...
1
vote
1answer
60 views

Prove that the Zariski space $\text{Zar} \space (K,A)$ is compact.

I posted part of the proof from Matsumura's Commutative Ring Theory. I got stuck in the last sentence where it says "Hence the intersection of all the elements of $\mathcal{A}$ is the same thing as ...
4
votes
0answers
69 views

Bass' paper on Gorenstein rings

I am currently reading the paper On the ubiquity of Gorenstein rings by Hyman Bass. I found difficulty to understand the proof of Proposition (7.2). Under the the following setting: $A$: commutative ...
0
votes
2answers
28 views

Let $F$ be a class of sets. Prove that $B - \mathop{\bigcup}_{A\in F} A = \mathop{\bigcap}_{A\in F} (B-A)$

Let $F$ be a class of sets. Prove that $B - \mathop{\bigcup}_{A\in F} A = \mathop{\bigcap}_{A\in F} (B-A)$ I've started like this: $X= B - \mathop{\bigcup}_{A\in F} A$ $Y= \mathop{\bigcap}_{A\in ...
1
vote
4answers
79 views

Prove that if $a<b$, then $-b<-a$

Prove that if $a<b$, then $-b<-a$ I'm a bit lost in this one, this is what I did: First case: $0<a<b$ $|a|<|b|$, so $|-a|<|-b|$ Since both are negative and $|-b|$ is greater ...
2
votes
1answer
147 views

How to remember all the proofs in mathematics

I have a problem where I forget the proof of a theorem after some time without reworking it out. However, my teacher said that he was able to prove a theorem even without reworking it out for a long ...
0
votes
1answer
43 views

Ten men selected to include a captain vs ten men selected to include at least one officer

I can't see how these are different scenarios, but the answers are different. The general scenario is that there is a company of volunteers that consists of a captain, a lieutenant, an ensign, and ...
0
votes
0answers
30 views

Help on Lagrange Error Calculation [duplicate]

Here is an example in Burden's Numerical Analysis book. My problem is in bold In example 2 we found the second Lagrange polynomial for $f(x)=1/x$ on $[2,4]$ using the nodes $x_0=2$, $x_1=2.75$, ...
2
votes
2answers
127 views

Confusion on the proof that there are “arbitrarily large gaps between successive primes”

I am trying to wrap my brain around a proof that proves that there are arbitrarily large gaps between successive primes. The proof is Given a natural number $N\ge2$, consider the sequence of $N$ ...
0
votes
2answers
40 views

How can the only maximal ideal of $C[x] / X^2$ be $(X)$?

In my notes I have the following example which I don't understand. Let $f$ be the canonical injection from $C$ to $C[X]/X^2$.The only maximal ideal of $C[X]/X^2$ is $(X)$ and $f^{-1}((X))$=$(0)$. ...
4
votes
1answer
75 views

Help needed to understand statements about torus

I am having trouble understanding two statements: Let $A$ be an algebraic curve in $\mathbb{P}^2$ over $\mathbb{C}.$ Consider its normalization $$\pi: \hat{A} \to A.$$ If genus $g(\hat{A})=1,$ ...
1
vote
2answers
93 views

Proof of the Intermediate Value Theorem [closed]

Theorem: Let $f$ be continuous on $[a,\,b]$ and assume $f(a)<f(b)$. Then for every $k$ such that $f(a)<k<f(b)$, there exists a $c\in[a,\,b]$ such that $f(c)=k$. proof: $f$ continuous at ...
3
votes
0answers
49 views

The proof of remainder theorem for polynomials: $f(\lambda)=0$ if and only if $x-\lambda$ divides $f$

From the paragraph starting with "Let us now suppose that $f(\lambda) = 0$" the author states that if $\deg(f) \leq 0 $ then $f=0$. But if $\deg(f) = 0$ then $f$ can be any constant, and not ...
0
votes
1answer
225 views

strong induction example

There is following example given in a book. I am not sure how do we conclude that $a$ is divisible by prime? See this section: Case 2 ($k + 1$ is not prime): In this case $k+1=ab$ where $a$ and ...
0
votes
2answers
56 views

Explanation of this proof of the principle of analytic continuation

I am confused about the last paragraph, He states that we choose a point $z_1$ inside the disk, and draw another disk around $z_1$ with radius $\delta_1$ Then says "f is identically zero on this ...
0
votes
1answer
35 views

Principle of analytical continuation proof confusion

Let $f$ be holomorphic on a region $D$ and assume I can find a sequence $\{w_k\}$ in $D$ s.t. $w_k \to z_0$ w/ $z_0 \in D$, $w_k \not = z_0$ & $f(w_k) = 0$. Then $f(z) = 0$ $\forall z \in D$ ...
1
vote
1answer
50 views

Fundamental Characterisation of Measureable Functions Explanation (Characterization)

I am just starting learning about measure theory (what a great way to spend Christmas...!), and I am unclear on this following claim: Characterisation of Measurable Functions. It appears to be a very ...
1
vote
3answers
82 views

Just a proof of algebra

If $a+b+c=0$, Show that $\left[\dfrac{a}{b-c}+\dfrac{b}{c-a}+\dfrac{c}{a-b}\right]\left[\dfrac{b-c}{a}+\dfrac{c-a}{b}+\dfrac{a-b}{c}\right]=9.$ I am struck with this problem but can't find a ...
2
votes
2answers
95 views

In Zagier's one-sentence proof, why is S defined to be {(x,y,z)∈ℕ^3:x^2+4yz=p,p prime}?

I've looked at a very clear explanation of Zagier's proof (specifically, it can be found here:http://danielmath.wordpress.com/2012/12/26/one-sentence-proof/) but the first step still eludes me: why is ...
0
votes
1answer
58 views

Localization of Rings: Show $R_{f} \simeq R[X]/\langle 1-fX \rangle$

Let $R$ be a ring, $f \in R$, and $X$ a variable. Show $R_{f} \simeq R[X]/\langle 1-fX \rangle$. I am a beginner in algebra and I am reading a textbook in commutative algebra. What I do not ...
0
votes
1answer
59 views

Explanation of Proof that field sum of more than 2 elements is 0. [duplicate]

"Suppose the field $F$ is finite. If $f\colon F\to F$ is any bijection, then we can conclude that $\sum_{x\in F}x=\sum_{x\in F}f(x)$. Let $\alpha\in F$ such that $\alpha\ne 0$. Then $x\mapsto \alpha ...