Is $\frak c = \aleph_1$?
64500
$\aleph_1 = 2^{\aleph_0}$
46946
Is there an elementary proof that $\sum \limits_{k=1}^n \frac1k$ is never an integer?
128673
Given $A : \mathcal{C} \rightarrow \mathcal{E}$, when does $\operatorname{colim} A = A(1)$?
61262
Is $\cot(z)-\frac1z$ bounded on a given circle?
4782
Can we distinguish $\aleph_0$ from $\aleph_1$ in Nature?
120010
Show that $\aleph_{\alpha}^{\aleph_{1}} = \aleph_{\alpha}^{\aleph_{0}}\cdot 2^{\aleph_1}$
20789
If $\Sigma \vdash \varphi$ then $\Sigma \vdash \forall x \varphi$. Why when $(\varphi \implies \forall x \varphi)$ is not true?
154001
$(g\circ f)^{-1}=f^{-1}\circ g^{-1}$
90475
Coordinate-free proof of $\operatorname{Tr}(AB)=\operatorname{Tr}(BA)$?
110103
Prove that there are $0<c_1 < c_2<1$ s.t. $\frac{1}{g'(c_1)}+\frac{1}{g'(c_2)}=2$ . The same for $g'(c_1)g'(c_2)=1$.
125668
A proof that BS(1,2) is not polycyclic
94469
How to prove that $|HK| = \dfrac{|H| \; |K|}{|H \cap K|}$?
135577
Prove the existence of $c$ such that $f'(c) = 2c(f(c) - f(0))$
72507
Does $ab=1$ imply $ba=1$ in a ring?
90845
Is this posible in GR $g_{ab}g^{ab}=1$?
16534
Can $(\Bbb N,\leq)$ have an $\aleph_0$-categorical theory (in a larger language)?
135511
$M_a =\{ f\in C[0,1] |\ f(a)=0 \}$ for $a$ $\in$ $[0,1]$. Is $M_a$ finitely generated in $C[0,1]$?