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Jan
14
comment Orientation preserving and orientation reserving of a parameterisation of a curve
$u'(t)\geqslant 0$ or $u'(t)\leqslant 0$, right?
Dec
5
comment Group Theory: Show that $G/Z(G) \cong Inn(G) $?
Use the first isomorphism theorem.
Nov
28
comment Count of relatively prime numbers up to some $m$
You can use the Inclusion-Exclusion principle. Have you seen that?
Nov
16
comment Characterize continuous functions $f : X → Y$ for which $f^{− 1} ( \{ a\} )$ is open
@Nex Yeah, I know :)
Nov
16
comment Characterize continuous functions $f : X → Y$ for which $f^{− 1} ( \{ a\} )$ is open
@Nex He's probably asumming points are closed in $Y$.
Nov
16
comment Characterize continuous functions $f : X → Y$ for which $f^{− 1} ( \{ a\} )$ is open
What happens if a set is both open and closed?
Nov
2
comment Solving a polynomial with complex coefficients
Let $w=z^3$. Solve $w^2+(2i-1)w-1-i$ and then find $z$.
Oct
18
comment determine the frontier
What do you mean by frontier? In any case, note that every real number is the limit of a sequence of irrational numbers.
Oct
18
answered Modulus of n-tuples?
Oct
18
revised Which kind of problems of Set Theory are solved in which way?
Delete set-theory tag
Oct
18
suggested approved edit on Which kind of problems of Set Theory are solved in which way?
Oct
12
comment Show that $\lim_{n\to \infty} (\sqrt{n+1} - \sqrt n) = 0$ using the definition of a limit
Which sign change? I just used that $a^2-b^2=(a-b)(a+b)$. For the limit, given any $\varepsilon$, you need to find $N$ such that $|\sqrt{n+1}-\sqrt n|<\varepsilon$ if $n>N$. If you find $N$ for $1/(2\sqrt n)$ you're done by what's above.
Oct
12
answered Show that $\lim_{n\to \infty} (\sqrt{n+1} - \sqrt n) = 0$ using the definition of a limit
Oct
4
answered Describing the image of the complex set $G = \{ z = x+iy \mid x^2 + y^2 + 2x + 2y + 1 = 0\}$
Oct
1
comment Prove that $\gcd(g_a,g_b) = 1$ given that for $n \in Z^{\geq 0}$, define $g_n = 2^{2^n} + 1$
math.stackexchange.com/questions/1458929/…
Sep
28
answered A locally injection is an injection?
Aug
7
comment General Isomorphism, for all algebraic structures
The "homomorphisms" in the category of topological spaces are the continuous functions, and they do not behave like homomorphisms in groups, rings,... See here.
Aug
7
comment General Isomorphism, for all algebraic structures
Take $f$ from $[0,2\pi)$ to the unit circle in $\Bbb R^2$ defined by $f(t)=(\cos(t),\sin(t))$.
Jul
20
comment Bounded Matrix-Vector Multiplication
$Ax$ is a vector, so its norm $\| Ax\|$ is a well-defined real number. If you are asking about the boundedness of the set $B=\{\| Ax\| :x\in \Bbb R^p\}$, then $B$ is bounded if and only if $A=0$.