Proof that a derived set is closed in a $T_1$-space
125962
The set $A'$ is closed in a $T_1$ space
3896
A closed set in $(A, \mathcal T_A)$ that is not closed in $(X, \mathcal T)$
153934
How to prove that a set R\Z is open
99341
Basic question: closure of irreducible sets
79213
Proof about a Topological space being arc connect
10180
How to prove that this set is closed?
24050
Characterization of the Closure of a Set
127900
Interior of closure of open set in topological space
143105
Show that, for every real number a, the set {$x ∈ S | f(x) ≤ a$} is a closed subset of $\mathbb R^d$
52014
Showing $(C[0,1], d_1)$ is not a complete metric space
47680
Prove that the union of the interior of a set and the boundary of the set is the closure of the set
45219
Proving that $C$ is a subset of $f^{-1}[f(C)]$
148992
A finite set is closed
122190
Kernel of $T$ is closed iff $T$ is continuous
34964
Recursive representation of closed sets in Baire Space ($\Sigma^1_1$)
18596
A set $A \subset l_1$ is compact if and only if closed, bounded, and one other condition
105445
Are the rationals a closed or open set in $\mathbb{R}$?