Questions about studying mathematics without formal instruction.

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2
votes
1answer
66 views

the diagonal $\Delta (Y) =\{(y,y)\in Y \times Y\}$ is closed in $\Bbb P^m \times \Bbb P^m $

Asumme that $\Pi : \Bbb P^n \times \Bbb P^m \rightarrow \Bbb P^m $is a closed map. $X\subset \Bbb P^n$ And $Y\subset P^m$ $f: X\rightarrow Y$ be a morphism of projective varieties. ...
0
votes
2answers
80 views

Every projective algebraic set can be written as the zero set of finitely many homogeneous polynomials of the same degree.

Definition: Let $I \subset k[x_0,\ldots,x_n]$ be a homogeneous ideal (or a set of homogeneous polynomials). The set $Z(I) := \{(a_0 : \cdots : a_n)\in P^n ; f(a_0,\ldots,a_n) = 0 \ \ \forall f \in ...
0
votes
0answers
82 views

Rings having the same characters but not isomorphic.

I want to show that these two rings have the same characters but they are not isomorphic for $\nu>2$ Thank you for helping. $$H=k+kt^{4\nu}(1+t)+kt^{6\nu}(1+t)+kt^{7\nu}(1+t)+k[[t]]t^{8\nu}$$ ...
38
votes
7answers
2k views

When working proof exercises from a textbook with no solutions manual, how do you know when your proof is sound/acceptable?

When working proof exercises from a textbook with no solutions manual, how do you know when your proof is sound/acceptable? Often times I "feel" as if I can write a proof to an exercise but most ...
7
votes
3answers
288 views

Is formal logic necessary for pure/“higher” mathematics?

I'm asking this as an autodidact who wants to learn math rigorously for its own sake. And I was just wondering if understanding proofs could be achieved without a formal grounding in symbolic logic. I ...
2
votes
0answers
62 views

“Teach yourself” guides [closed]

I really liked Teach Yourself Logic: A Study Guide by the user Peter Smith. It is a thorough guide how to teach yourself logic and set-theory from scratch up to any level with book recommendations for ...
2
votes
2answers
89 views

Second Course in Number Theory - Self Study

I just finished a first course in number theory using Dudley's Elementary Number Theory. This was by far my favorite math course and I want to learn more number theory this summer. As far as ...
1
vote
1answer
47 views

Differential forms and wedge product exercise.

Show that $$\omega \wedge v(\left <a_1,a_2,a_3 \right>,\left <b_1,b_2,b_3 \right>) = c_1 dx\wedge dy + c_2 dx\wedge dz + c_3 dy \wedge dz.$$ I wasn't given the form of $\omega$ or ...
10
votes
0answers
126 views

Soft question: How does basic differential geometry “fit together”?

I'm self-studying diff geom from Lee's Introduction to Smooth Manifolds. He warns the reader that there's a lot of machinery to construct, which is fine, and he explains things with wonderful clarity. ...
1
vote
1answer
18 views

“And”/“or” in solutions of inequalities

I'm working on introductory chapters of Lang's "A First Course in Calculus", and in the part on inequalities I didn't quite understand why, when the solution of an inequality consists of more than one ...
2
votes
1answer
24 views

Minimum distance from origin in $\mathbb{R}^3$ but Hessian is indefinite

Problem: Consider the Set $M= \lbrace (x,y,z) \in \mathbb{R}^3 \mid x^2+2y^2-z^2=1 \rbrace$ and find all Points on $M$ which have minimal euclidian distance from the origin. My approach: I ...
1
vote
1answer
19 views

Bayes with conditional independence

I have a problem that I can't work out I've two conditional independent A,B such as $P(A,B|C) = P(A|C)P(B|C)$ Now I've to find posterior formula for: $P(C | A,B)$, now what I got was pretty ...
2
votes
1answer
90 views

Prove the Gaussian curvature $K=0$

If two families of a geodesics on a surface intersect at a constant angle $\theta$, prove that the Gaussian curvature of the surface is zero, i.e. $K=0$. Please explain how to show the question. ...
2
votes
3answers
24 views

Find a vector orthogonal to other two given and ends at a plane

I am reviewing Calculus III using Mahavier, W. Ted's material and get stuck on one question in chapter 1. Here is the problem: Assume $\vec{u},\vec{v}\in \mathbb{R}^3$. Find a vector ...
4
votes
0answers
68 views

Strategy for self-studying after M.S.

For someone who has finished their M.S. degree in pure mathematics, what is a good way to keep learning mathematics within your specialization? Would you suggest reading research articles from ...
1
vote
2answers
37 views

Problem of continuous function

Define the function $g(x) = x^2\cos\frac1x$ for $x\ne 0$. What should be the value of $g(0)$ if $g(x)$ is a continuous function? Explain your work and justify your answer. Frankly, I have no ...
1
vote
2answers
65 views

Third and average price auction

Third price auction: the winner is the highst bidder but this time instead of paying the second highst bid, he would pay the third highst bid. -assume there are at least 3 bidders. - Average price ...
0
votes
1answer
49 views

Prove the assertion on the game theory.

if a dominant strategy for player1 is added to finite normal form game then the payoff to player1 at any equlibrium of the new game must be at least as great as any nash equlibrium payoff ...
0
votes
3answers
61 views

How do I find $\lim_{x\rightarrow \infty} x\sin \frac {c}{x}$?

How do I find the following limit for some real $ c $? $$\lim_{x\rightarrow \infty} x\sin \frac {c}{x} $$
0
votes
0answers
18 views

Polar coordinates: Slope of tangent

Would anyone mind telling me how to solve this problem? It seems strange as my answer is $-1$. Do I have to apply this formula, $(r'sinθ +rcosθ)/(r'cosθ -rsinθ)$ ?
0
votes
2answers
31 views

Need help with this absolute value equation

I need to solve the following equation involving absolute value: $$|x-1| = 1-x$$ Looking at the term $x-1$, I thought I'd divide the interval into parts: $x < 1$ and $x \geq 1$. Now, when ...
2
votes
1answer
58 views

Misuse of Tangent Vector

I am quite confused with the term tangent used in differential geometry books. It seems to me that people use this word quite loosely. For example, one definition about tangent space in my book is as ...
0
votes
1answer
29 views

Series representation of cosh or the hyperbolic cosine. [closed]

What is the Series representation of cosh or so-called the hyperbolic cosine? Can you help me guys?
2
votes
2answers
34 views

Question in relation to Fundamental Theorem of Calculus

I obtain two answers, one is $\dfrac1{\sqrt{1+x^6}}$ and another one is $\dfrac{2x}{\sqrt{1+x^{12}}}$ by using Fundamental Theorem of Calculus, but I am not so sure. Would anyone help me?
1
vote
1answer
46 views

Washer method and shell method

(1) Sketch the region enclosed between the curve $y=sin^2x$ and the straight line $y =2x/π$ (2) Find the volume of the solid $S$ obtained by revolving the region in part (1) about the $y$-axis by ...
1
vote
2answers
184 views

Partitions of Natural Numbers [duplicate]

This is a question from Complex analysis by Stein. The question is Prove that it is not possible to partition $\mathbb N$ into finitely many infinite AP's with distinct common differences.(other ...
1
vote
3answers
48 views

Definite integrals: Calculating Volume

Suppose $D$ is the region in the $xy$-plane bounded by the parabola $y=x^2$ and the line $y=2x$. Find the volume of the solid generated by rotating $D$ about 1) $x$-axis 2) $y$-axis. Are the ...
1
vote
1answer
53 views

Calculus: Application of definite integrals

Suppose $a>0$ is a constant. Let $C$ be the curve $y=\cosh x$, for $-a \leq x\leq a$. Let $D$ be the region bounded by $C$, $|x| = a$ and the $x$-axis. 1) Find the length of $C$ 2) Find the area ...
1
vote
2answers
35 views

Proving this binomial identity

I'm required to prove the following binomial identity: $$\sum\limits_{k=0}^l {n \choose k} {m \choose l-k} = {n+m \choose l}$$ I tried various arrangements but reached nowhere. Finally I turned to ...
0
votes
1answer
22 views

Every metrizable space with a countable dense subset has a countable basis

I'm working on this problem from Munkres: Show that every metrizable space with a countable dense subset has a countable basis. Here's my attempt at a proof. Let $X$ be a metrizable space with ...
0
votes
0answers
41 views

Full Course in Mathematics

I'm in a bit of a bind. I've recently taken up a degree in Computer Science, and it bypasses much of the mathematics. I picked this course as it's been years since I've exercised any math beyond basic ...
5
votes
3answers
234 views

Laplace transforms: Convolution

Find $$1*1*1*\cdots*1\quad n\,\,\text{ factors}$$ that is, a function $f(t)=1$ convolution with itself for a total of $n$ factors. Would anyone mind helping me? I have no idea what I should do. ...
2
votes
2answers
30 views

Two sequences are equivalent. Prove that one is Cauchy iff the other is Cauchy.

This question has already been asked and answered here Let $ϵ>0$ be given. With loss of generality, we may assume $ϵ$ is rational. Suppose $a_n$ is a Cauchy sequence and $b_n, a_n$ are ...
2
votes
0answers
46 views

Continuity of the right-hand derivative of a Convex function (help with the proof)

Hi everyone I have some trouble with one point in the following proof. Let $f$ be a convex function (strict convex function) on a real interval. If $f'_-(a)=f'_+(a)$ where $f'_-$ and $f'_+$ are ...
2
votes
1answer
72 views

Laplace transformation: second shifting theorem

I know the answer is $1/(s^2) +e^-6s (2/s^3 -14/s -1/s^2 )$, but can anyone tell me how to evaluate the solution? I really get stuck.
3
votes
0answers
123 views

Isolation and self-study

A little background: I am currently a sophomore (studying mathematics) at an unknown university in the Middle East. My mother is European so it does not make sense to study mathematics in the Middle ...
0
votes
1answer
21 views

Why is this a boolean algebra

Let $A = \{a,b\}$. The $\mathcal P(A) = \{\emptyset,\{a\},\{b\},A\}$. Let $+$ be $\cup$, $\cdot$ be $\cap$, complement be set complement, $1$ be $A$, and $0$ be $\emptyset$. I need to explain why ...
0
votes
1answer
28 views

Conditional CDF

$X$ and $Y$ are independent and uniformly distributed on $[0,1]$. $Z=arctan(\frac{Y}{X})$ and $Z$ is restricted between $[0,2\pi)$. Given that $A=0 \leq Z\le \pi/4$ , what is the conditional CDF of ...
1
vote
2answers
41 views

strict midpoint convex $\Rightarrow$ strict convex (help with a proof)

Hi everyone I have trouble with the following I think is something very simple, but I cannot figure out yet the correct approach for the strict inequality If $f$ is continuous and $f$ is strict ...
2
votes
3answers
66 views

CDF of $max(X,X^2)$

$X$ is uniformly distributed on $[-1,1]$. And $Y=max(X,X^2)$. What is $F_{Y}(t)$ , the CDF of $Y$? My attempt: I tried to graph it, but I think I found wrong. I found the joint pdf $5/6$. Is this ...
0
votes
0answers
38 views

Describe the language generated by the grammar $G = \{\Sigma, \Delta\ S, I \} $

I need to describe the language generated by the grammar $G = \{\Sigma, \Delta\ S, I \} $, where $$\Sigma=\{0,1\epsilon \}, \Delta = \{S,X,Y,Z\}$$ and $$I = \{S \to0X|1Y, x \to1Y|1Z, Y \to0X|0Z, Z ...
2
votes
3answers
168 views

Any good introductory book/tutorial on Fourier Transform (up to FFT) with plenty of exercises and solutions?

I wonder what could be a good book to start learning in depth all aspects of the Fourier transform up to the FFT algorithm, and beyond. I am going to dedicate quite some time on the subject, so I ...
0
votes
1answer
19 views

Confused about a proposition about rational polynomial

I'm studing complex analysis.I saw that $R(z)$ is a rational function.Consider the function $R(1/z)$ which we can rewrite as a rational function $R_1(z)$,and set $R(\infty)=R_1(0).$ with the ...
1
vote
2answers
72 views

why $\infty$ +$\infty$, $\infty$ - $\infty$ and 0⋅$\infty$ are left undefined.

I'm reading http://en.wikipedia.org/wiki/Riemann_sphere, and having the following question. 1. What's the mean of symbol $\infty$?Is it a surreal number? 2. they write note that ∞ + ∞, ∞ - ...
1
vote
0answers
31 views

Create a finite-state machine

I need to create a finite-state machine which accepts strings whose characters are in {a,b,c} and produce output strings of T's and F's. The machine outputs a T once the characters ab is encountered ...
0
votes
1answer
56 views

Uniqueness of best approximation in strictly or uniformly convex normed linear spaces

I am studying approximation theory, just to learn basically, and in different books I saw a theorem about uniqueness of best approximation. One book uses strict, the other uses uniform convexity with ...
1
vote
1answer
30 views

A doubt regarding splitting field.

$\Bbb Q(\omega)= \Bbb Q(\sqrt3,\iota)$ This is written in my text book that i am following. But I think this is a typo. Since $\Bbb Q(\sqrt3,\iota)$ is a larger field. in which $\Bbb Q(\omega)$ is ...
0
votes
1answer
15 views

Deriving Stochastic Euler Equation

If a consumer has utility function \begin{equation*} u(c_t) = ac_t - \cfrac{b}{2}c_t^2 \end{equation*} and present value budget constraint \begin{equation*} \sum_{j=0}^\infty E_t[\beta^jc_{t+j}] = ...
0
votes
0answers
20 views

symmetric normalized Graph Laplacian and symmetric normalized Adjacency matrix share the same eigenvalue

I am trying to show that the symmetric normalized Graph Laplacian and symmetric normalized Adjacency matrix share the same eigenvalues ($\lambda_i$ and $1 - \lambda_i$ for i=1 to n. $\lambda$ is an ...
3
votes
1answer
72 views

Application of Picard-Lindelöf to determine uniqueness of a solution to an IVP

I am still struggling quite a lot with the Picard-Lindelöf-Theorem (also known as the Cauchy-Lipschitz-Theorem). Problem: Consider the following IVP with $\alpha \neq 1$ $$\begin{cases} y'&= ...