Give a counterexample of $K_{1}$ and $K_{2}$ closed, but that $A$ is not closed.
28683
Sum of two closed sets in $\mathbb R$ is closed?
7607
How to prove that $\mathbb{Q}$ is not the intersection of a countable collection of open sets.
149095
$f(\mathbb{R}\setminus \mathbb{Q}) \subseteq \mathbb{Q}$ and $f(\mathbb{Q}) \subseteq \mathbb{R}\setminus \mathbb{Q}$ imply that $f$ is not continuous
77297
Counterexample for $A,B\triangleleft G,G/A\cong B \Rightarrow G/B\cong A$
125043
Show that F(A−B) is not necessarily a subset of F(A)−F(B).
48162
Is the subset $[0, \sqrt2] ∩\mathbb{Q} ⊂ \mathbb{Q}$ closed, bounded, compact?
70959
Prove that a set $E$ is closed iff it's complement $E^{c}$ is open
8015
Compact set plus a closed set is a closed set
125962
The set $A'$ is closed in a $T_1$ space
95244
Prove that a finite union of closed sets is also closed
140962
Counterexamples for $G \times H$ isomorphic to $G$ implies $H$ trivial.
22131
Prove if nonempty $K \subset\mathbb{R}$ is compact, then both sup $K$ and inf $K$ exist and are elements of $K$.
115272
Showing that projections $\mathbb{R}^2 \to \mathbb{R}$ are not closed
81927
Example of closed and bounded in $\mathbb Q$ set that is not compact
10180
How to prove that this set is closed?
128482
The set of subsequential limits is closed
78117
Closed property of nonempty finite set