Showing that $S_1 \cup S_2$ is countable
18223
Prove that the union of two disjoint countable sets is countable
16649
Topological space with countable open cover $\{U_\alpha\} $ with each $U_\alpha$ second-countable, is second countable
4111
Outer measure is countably subadditive
68139
Show that the set of polynomials with rational coefficients is countable.
137707
How to prove that $(A\cup B) \cap C \subseteq A\cup (B\cap C)$
134844
Summablity and countability
127720
Prove that $\bigcup_{n \in \Bbb{N}} [1/n,1] = (0,1]$.
10909
$|A \cup B| = \mathfrak{c}$, then $A$ or $B$ has cardinality $\mathfrak{c}$.
48174
Are countable topological spaces second-countable?
78763
Prove that $ S=\{0\}\cup\left(\bigcup_{n=0}^{\infty} \{\frac{1}{n}\}\right)$ is a compact set in $\mathbb{R}$.
99656
Prove that the set of all algebraic numbers is countable
63276
Is an arbitrary union of a chain of countable sets countable?
44974
Infinite set and countable subsets
36315
Is $[0,1]$ a countable disjoint union of closed sets?
139247
Real Analysis - Prove a set is countable
30770
Prove that $C\left(\bigcup_{\alpha\in I} A_\alpha\right)=\bigcap_{\alpha\in I}C\left(A_\alpha\right)$
99478
countably infinite union of countably infinite sets is countable