Open sets in Q wrt euclidean topology.
15480
Open set in a metric space is union of closed sets
127900
Interior of closure of open set in topological space
90782
Examples of topologies in which all open sets are regular?
75829
Dimension of open subsets of $R^n$
126463
Can open sets and closed sets of $\mathbb{R}^n$ be homeomorphic?
64861
Intersection of open affines can be covered by open sets distinguished in *both*affines
63442
Show that if $U$ is an open connected subspace of $\mathbb{R}^2$, then $U$ is path connected
41690
Products of Continuous functions: Topology
135025
Axiomatization of a Topology using the Boundary
27998
Prove existence of disjoint open sets containing disjoint closed sets in a topology induced by a metric.
92639
Prove that the family of open sets in $\mathbb{R}$ has cardinality equal to $2^{\aleph_0}$
17607
Open maps which are not continuous
153934
How to prove that a set R\Z is open
77563
Homeomorphism between spaces equipped with cofinite topologies
150796
Subspace topology and order topology (2)
138202
Every open set in $\mathbb{R}$ is a countable union of closed sets
46788
Compact open topology on space of continuous maps from a discrete space is a product