- Prove that the set $F=\big\{x\in X\;|\; f(x)=g(x) \big\}$ is closed set of $X$.
- If $D$ is a dense subset of $X$, such that $f (x) = g(x)$, for every $x\in D$, prove that $f = g$.

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## Search found 307 matches

- Sun Mar 31, 2019 8:44 am
- Forum: General Topology
- Topic: Continuous functions
- Replies:
**0** - Views:
**1060**

### Continuous functions

Let $(X,\rho)$, $(Y,d)$ two metric spaces and $f,g:X\longrightarrow Y$ two continuous functions.

- Sat Mar 30, 2019 2:36 pm
- Forum: General Topology
- Topic: Not closet set
- Replies:
**0** - Views:
**918**

### Not closet set

Let $(X,\rho)$ a metric space and $(x_n)_{n\in{\mathbb{N}}}$ a Cauchy sequence in $X$, such that the set $\{x_n\;|\; n\in{\mathbb{N}}\}$ of the terms of this sequence it isn't a closed set. Prove that exists $x\in X$, such that $x_n\stackrel{\rho}{\longrightarrow}x$.

- Sat Mar 30, 2019 8:43 am
- Forum: General Topology
- Topic: Two examples
- Replies:
**0** - Views:
**923**

### Two examples

Give an example of a descending sequence $(F_n)_{n\in\mathbb{N}}$ of non-empty closed subsets of metric space $(\mathbb{R}, |\cdot|)$, such that $\bigcap_{n=1}^{\infty}F_n=\varnothing$. Give an example of a descending sequence $(F_n)_{n\in\mathbb{N}}$ of non-empty closed subsets of metric space $(\...

- Fri Nov 16, 2018 11:33 am
- Forum: Algebraic Structures
- Topic: Symetry group of Tetrahedron
- Replies:
**2** - Views:
**611**

### Re: Symetry group of Tetrahedron

We give a solution in the case of the tetrahedron: Definition: A symmetry of a (regular) tetrahedron $S$ is a linear transformation $T:\mathbb{R}^3\longrightarrow\mathbb{R}^3$ with orthogonal matrix which also leaves tetrahedron $S$ unchanged(*), i.e. $T(S)=S$. Definition: An axis of symmetry of a ...

- Fri Aug 31, 2018 5:53 pm
- Forum: Multivariate Calculus
- Topic: Area & surface integral
- Replies:
**0** - Views:
**372**

### Area & surface integral

Let $E$ be the surface with parametric representation \begin{align*} \overline{R}:(-3,3)&\times[0,2\pi]\longrightarrow{\mathbb{R}}^3\,; \quad \overline{R}(r,\theta)=\left({\begin{array}{c} \frac{r}{\sqrt{9-r^2}}\,\cos{\theta}\\ \frac{r}{\sqrt{9-r^2}}\,\sin{\theta}\\ \theta \end{array}}\right)\,, \en...

- Fri Aug 31, 2018 5:24 pm
- Forum: Multivariate Calculus
- Topic: Surface area of an Elliptic Paraboloid
- Replies:
**1** - Views:
**388**

### Re: Surface area of an Elliptic Paraboloid

... $$A_P=ab\int_0^1\int_0^{2\pi} \sqrt{1+\frac{4r^2\cos^2\theta}{a}+\frac{4r^2\sin^2\theta}{b}}\,r\,d\theta dr$$ ... The integral $\int_0^{2\pi} \sqrt{1+\frac{4r^2\cos^2\theta}{a}+\frac{4r^2\sin^2\theta}{b}}\,d\theta$ is an elliptic integral of second type. Thus, the corresponding double integral ...

- Fri Aug 31, 2018 4:23 pm
- Forum: Multivariate Calculus
- Topic: Volume, area & line integrals
- Replies:
**0** - Views:
**392**

### Volume, area & line integrals

Let $E$ be the surface with parametric representation \begin{align*} \overline{R}:[0,6)&\times[0,2\pi]\longrightarrow{\mathbb{R}}^3\,; \quad \overline{R}(r,\theta)=\left({\begin{array}{c} \frac{r^2}{\sqrt{36-r^2}}\,\cos{\theta}\\ \frac{r^2}{\sqrt{36-r^2}}\,\sin{\theta}\\ r \end{array}}\right)\,, \en...

- Tue Aug 14, 2018 6:41 am
- Forum: Multivariate Calculus
- Topic: Show that a vector field is not conservative (example)
- Replies:
**4** - Views:
**755**

### Re: Show that a vector field is not conservative (example)

Sure, in this case! But in general, to find a suitable curve isn't easy.andrew.tzeva wrote:Thank you. The 2nd solution (with the direct counter-example) is much more helpful.

- Sun Aug 12, 2018 11:01 am
- Forum: Multivariate Calculus
- Topic: Show that a vector field is not conservative (example)
- Replies:
**4** - Views:
**755**

### Re: Show that a vector field is not conservative (example)

...I tried using $r(t)=t\vec{i}+t\vec{j}, \space t\in[\alpha,\beta]$, but it didn't work. What curve would be a better choice for $C$ and what's the deal with $\mathrm{rot}\,F$ being zero?.. Here is a 2nd solution, choosing an appropriate (closed) curve: The line integral of $\overline{F}$ over the...

- Sat Aug 11, 2018 10:29 am
- Forum: Multivariate Calculus
- Topic: Show that a vector field is not conservative (example)
- Replies:
**4** - Views:
**755**

### Re: Show that a vector field is not conservative (example)

First we write down a useful theorem: If a continuously differentiable vector field $\overline{F}:U\subseteq{\mathbb{R}}^n\longrightarrow{\mathbb{R}}^n\,,$ where $U$ is open, is conservative, then, for every $\overline{x}\in U$, the Jacobian matrix ${\bf{D}}\overline{F}(\overline{x})$ of $\overline{...