Open subset of a plane
51048
How can I show that if the second fundamental form of a surface is identically equal to zero, then the surface is a plane?
46469
$n$ Lines in the Plane
64519
Subsets of a list
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 $\mathbb R^1$
93623
Showing that every connected open set in a locally path connected space is path connected
88823
How can the empty set be open in the definition of the cofinite topology?
110957
No non-trivial clopen sets in $\mathbb{R}$? How to give a direct proof?
70264
Prove that the inverse image of an open set is open
117020
Regular $n$-gon in the plane with vertices on integers?
152976
Real Analysis: open and closed sets
59424
Showing that $B=\{(x,y) \in \Bbb{R^2} \mid \ xy>0 \}$ is an open set
56728
Question about limit points of a Subset of $\mathbb{R}$
72468
Open and closed balls in $C[a,b]$
74330
$\mathbb{R}^n$ not homeomorphic to open subset of $\mathbb{R}^m$
123897
Sum of two open sets is open?
123939
N circles in the plane
146204
Subsets sharing at most $i$ elements