Babak S.

Age: 41

Counting is a less precise tool for infinite sets than for finite ones. The shepherdess who can count her flock of a hundred sheep will know if the wolf has taken one; but, if she has an infinite flock, she won't notice until almost all of her sheep have been lost.

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Peter J. Cameron

All I am doing here as hints or answers, are dedicated to him.

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42m	comment	If $d(x,A)=0\forall x\in X$ for some subset $A$ of $X$, does it follow that $A$ is dense? You mean $X$ and $A$ can not be disjoint?
47m	comment	$\int_{0}^{\pi/2} \text{arctanh}(\sin x) \text{arctan}(a \tan(x)) \cos(x) \ dx$ Which problem led you to get this integral?
53m	reviewed	Approve suggested edit on similarity of triangles and tangents
1h	reviewed	Approve suggested edit on Find the limit without use of L'Hôpital or Taylor series: $\lim \limits_{x\rightarrow 0} \left(\frac{1}{x^2}-\frac{1}{\sin^2 x}\right)$
1h	reviewed	Approve suggested edit on Find the limit without use of L'Hôpital or Taylor series: $\lim \limits_{x\rightarrow 0} \left(\frac{1}{x^2}-\frac{1}{\sin^2 x}\right)$
1h	comment	How to solve $at + b = 0 \pmod {(a-t)}$? Nice Maisam. +1
1h	comment	Polynomial discrete mathematics +1 ${}{}{}{}{}$
1h	comment	Commutator property Make it as an comment to @Don's leading answer, if you want him to have a looking at it.
1h	comment	Order 7 matrix with odd entries has determinant a multiple of 64? $3^i, i=2k$ mode $4$
1h	comment	Cardinality of Center of p Groups $ \|Z(G)\| \neq p^{n-1}$ @MaisamHedyelloo: Thanks maiysam. Elthomas 2A.
1h	comment	Cardinality of Center of p Groups $ \|Z(G)\| \neq p^{n-1}$ @amWhy: I am glad cause of that. This is more precious to me that I can make someone around the World to have a good feeling. This shows that $0$ (it is me) is not nothing. :-)
1h	comment	$\lim_{x\to \pi/2} \;\frac 1{\sec x+ \tan x}$ +1 nice and complete.
1h	comment	Why $\operatorname{rank}(A^* A)=\operatorname{rank}(A)$ is equivalent to $A^* Ax=0$ if and only if $Ax=0$? +1 and happy eid. Eltemase 2a. ;-)
1h	reviewed	Approve suggested edit on Proving a relation between $\sum\frac{1}{(2n-1)^2}$ and $\sum \frac{1}{n^2}$
1h	comment	Divergence of $\sum_{n=2}^\infty\frac{1}{(\ln n)^x}$ for $x>1$ @Gary: See this one math.stackexchange.com/a/398964/8581. :-)
1h	comment	Notation of Planes @amWhy: :-o ${}$
11h	revised	Being inside or outside of an ellipse edited tags
11h	comment	Being inside or outside of an ellipse @Bedi: In fact on a line like $L$, $\|OA\|<\|OP\|$ which $P$ is the intersection of $L$ with ellipse.
11h	reviewed	Approve suggested edit on Show that $\alpha_1u+\alpha_2v+\alpha_3w=0\Rightarrow\alpha_1=\alpha_2=\alpha_3=0$
11h	answered	Being inside or outside of an ellipse