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visits member for 4 years, 1 month
seen 22 hours ago

Sep
9
answered Does finite equivalence classes implies that the set itself is finite.
Sep
2
comment Problem showing that $\partial D = \emptyset$
Suppose you had a point $x$ in the closure of $D$ but not in $D$ itself. What that would mean in terms of distances between $x$ and points of $D$?
Sep
1
comment Find a bijection to show $\left|B\right| = \mathfrak{c}$.
You use the same symbol $A$ for the set to be defined and for the definition
Aug
31
comment Judge the convergence of $\sum_{n=0}^\infty 1/\sqrt{n}$
Yes, but you can't use $n$ both in the RHS and as running variable in the sum!
Aug
31
comment Judge the convergence of $\sum_{n=0}^\infty 1/\sqrt{n}$
You mean $\sum_{k=1}^n\frac1{\sqrt{k}}\geq\sqrt{n}$?
Aug
30
comment An element of $SL(2,\mathbb{R})$
Suppose that $\left(\begin{array}{cc}a & b \\c & d\end{array}\right)\binom i1=\lambda\binom i1$ for some $\lambda\in\Bbb C$. Then $\frac{ai+b}{ci+d}=\frac{\lambda i}\lambda=i$. This yelds $a=d$ and $b=-c$. Combining this with $ad-bc=1$ you get $a^2+b^2=1$ so that $a=\cos t$, $b=\sin t$ for some $t\in \Bbb R$. But then your original matrix is rotation by $t$.
Aug
29
comment An element of $SL(2,\mathbb{R})$
That can be easily obtained just writing it down what it means.
Aug
28
revised An element of $SL(2,\mathbb{R})$
edited tags
Aug
28
answered An element of $SL(2,\mathbb{R})$
Aug
26
answered What is the relationship between the trace/norm of a quaternion and the definition in field theory?
Aug
23
comment Defining a partial order on $A\times B$, given partial orders on $A$ and on $B$
What is the problem in following the definition?
Aug
22
comment Show that sum of divisors of a composite number $n$ is $> n+ \sqrt{n}$
You already did
Aug
22
comment Show that sum of divisors of a composite number $n$ is $> n+ \sqrt{n}$
Is $n$ a divisor of $n$? Must be, or else the assertion is false (take $n=4$). Hence .....
Aug
20
comment How to find a 4D vector perpendicular to 3 other 4D vectors?
Well, of course you can define a $4$-dimensional cross-product by mimicking the usual (3-D) definition. The point is how to compute it quickly.
Aug
20
answered How to find a 4D vector perpendicular to 3 other 4D vectors?
Aug
20
comment An epimorphism from $\mathbb Z⊕\mathbb Z⊕\cdots$ to $\mathbb Q$
Right, but he's exactly asking for the construction .....
Aug
20
comment An epimorphism from $\mathbb Z⊕\mathbb Z⊕\cdots$ to $\mathbb Q$
The OP's asking for a map of additive groups, I suppose.
Aug
20
comment An epimorphism from $\mathbb Z⊕\mathbb Z⊕\cdots$ to $\mathbb Q$
How do you get, say, $\frac12$?
Aug
20
answered An epimorphism from $\mathbb Z⊕\mathbb Z⊕\cdots$ to $\mathbb Q$
Aug
20
answered If $p\mid|G|$ then how many elements of order $p$ are there in $G$?