Find a set A of rational numbers that is isomorphic to sup{$\omega$, $\omega^\omega$, $\omega^{\omega^\omega}$, $\cdots$}
106178
Question of an isomorphism of $\epsilon_ 0$ and a subset of the rationals.
3801
Functions that take rationals to rationals
146356
To prove two sets are isomorphic is it sufficient to find 2 injections or 2 surjections?
121831
Proving that rational numbers are dense
30105
Multiples of an irrational number forming a dense subset
12404
Union of definable sets
127242
$|A\times B|= \text{max}(|A|,|B|)$ for infinite sets
121685
Is there a bijection of the natural numbers which swaps $\frac{1}{n}$-summable subsets with $\frac{1}{\sqrt{n}}$-summable subsets?
132068
Write the set $\{1, 6, 11, 16,\ldots\}$ in compact (concise) notation.
148516
Proof that $\omega^\omega$ is completely metrizable and second countable
154230
In $\mathsf{ZF}$, for each set $A$ exists an ordinal $\alpha$ such that no surjection $A\to \alpha$ exists
130542
Every natural number $n$ can be written as $n=s-t$ with $\omega(s)=\omega(t)$
86542
Group of positive rationals under multiplication not isomorphic to group of rationals
129813
Is there an easy proof for ${\aleph_\omega} ^ {\aleph_1} = {2}^{\aleph_1}\cdot{\aleph_\omega}^{\aleph_0}$?
12701
Perfect set without rationals
93182
Constructing $g$ holomorphic differentials on $\omega^3=(z-1) \cdots (z-8)$
55978
Prove that for every countable ordinal $\alpha$, $\alpha\times[0,1)$ is order isomoprhic to $[0,1)$