Search found 308 matches
- Tue Mar 19, 2024 5:34 am
- Forum: Algebraic Topology
- Topic: Homotopy equivalence of closed curves
- Replies: 0
- Views: 3942
Homotopy equivalence of closed curves
Consider the family \[C_{\alpha}:\big({2(\alpha-\cos{t})\,\cos{t},\,2(\alpha-\cos{t})\,\sin{t}}\big)\,,\;t\in[0,2\pi]\,,\;\alpha\in\mathbb{R},\] of closed parametric curves. Let $X_{\alpha}$ is the image set of $C_{\alpha}$ equipped with the induced topology of $\mathbb{R}^2$. For which values of $\...
- Sun Mar 31, 2019 8:44 am
- Forum: General Topology
- Topic: Continuous functions
- Replies: 0
- Views: 14081
Continuous functions
Let $(X,\rho)$, $(Y,d)$ two metric spaces and $f,g:X\longrightarrow Y$ two continuous functions.
- 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 $f = g$.
- Sat Mar 30, 2019 2:36 pm
- Forum: General Topology
- Topic: Not closet set
- Replies: 0
- Views: 10048
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: 9980
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: 6233
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: 5601
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)\,,...
- Fri Aug 31, 2018 5:24 pm
- Forum: Multivariate Calculus
- Topic: Surface area of an Elliptic Paraboloid
- Replies: 1
- Views: 4748
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: 4116
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)\,,...
- Tue Aug 14, 2018 6:41 am
- Forum: Multivariate Calculus
- Topic: Show that a vector field is not conservative (example)
- Replies: 4
- Views: 7398
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: 7398
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...