How the each set $[-1,0)$ , $(0,1]$ are open in Y?
73130
A simple question about open set
144362
The construction of a Vitali set
63717
Is the closure of a set equal to the closure of the closure of that set?
123897
Sum of two open sets is open?
24050
Characterization of the Closure of a Set
21431
The set of all functions from $\mathbb{N} \to \{0, 1\}$ is uncountable?
93750
Can a set be neither open nor closed?
116806
Find the weak sequential closure of a set in $L^2(-\pi,\pi)$
3101
Meaning of a set in the exponent
116009
Proving that if a set is both open and closed then it is equal to the real numbers
56728
Question about limit points of a Subset of $\mathbb{R}$
70264
Prove that the inverse image of an open set is open
146664
The union of two linearly independent subsets
127900
Interior of closure of open set in topological space
125962
The set $A'$ is closed in a $T_1$ space
153934
How to prove that a set R\Z is open
43535
show that the interval of the form $[0,a)$ or $(a, 1]$ is open set in metric subspace $[0,1]$ but not open in $\mathbb R^1$