Why is $|HK| = \frac{|H||K|}{|H \cap K|}$?
5042
Order of a product of subgroups. Prove that $o(HK) = \frac{o(H)o(K)}{o(H \cap K)}$.
144521
Why does Möbius transformation $f(z)$ map the unit disk $|z|<1$ to itself?
3356
Why $\sum_{k=0}^m x^k $ is equal to $\dfrac{x^{m+1}-1}{x-1} $?
20317
Proving $\sum_{k=1}^{n}{(-1)^{k+1} {{n}\choose{k}}\frac{1}{k}=H_n}$
13274
Why does \cal{M}_n give “M>”?
32965
Proving that for infinite $\kappa$, $|[\kappa]^\lambda|=\kappa^\lambda$
108921
By substituting $z = h(t)$ show that $\delta(h(t))=\sum\limits_{i}\frac{\delta(t−t_i)}{\mid h^{\prime}(t_i)\mid}$
15441
Show $\frac{1}{4}\leq \mu(A\cap B \cap C)$
28350
$\lambda=\frac{2h}{p}$ instead of $\lambda=\frac{h}{p}$?
110643
What are the strategies I can use to prove $f^{-1}(S \cap T) = f^{-1}(S) \cap f^{-1}(T)$?
78471
Closure of $\{\frac{1}{n}\}:=M$
15312
Show that $R \cap R^{−1}$ is an equivalence relation on $X$?
127000
When does $V = \ker(f) \oplus \operatorname{im}(f)$?
64485
Proving $\displaystyle\lim_{h\to0}\frac{f(x+h)-2f(x)+f(x-h)}{h^2}=f''(x)$
89533
$\mathcal{K}(L^2(\mathbb{R}^m \times \mathbb{R}^n)) = \mathcal{K}(L^2(\mathbb{R}^m)) \otimes \mathcal{K}(L^2(\mathbb{R}^n))$?
14576
$(U\cap W)^0 = U^0 + W^0$
104753
$\sum_{k=1}^nH_k = (n+1)H_n-n$. Why?