« first day (1737 days earlier)      last day (2604 days later) » 

Martin Sleziak
Martin Sleziak
5:39 AM
specific-question It seems that Is a sphere a closed set? does not ask about closed in the sense closed subset of a topological space, but rather about closed surfaces or something similar. What tags should be added to reflect this. (Although I would leave general-topology, since this is mentioned in the question, too.) Would [tag:differential-geometry be a good fit?
5
Q: Is a sphere a closed set?

TomThe unit sphere in $\mathbb{R}^3$ is $\{(x,y,z) : x^2 + y^2 + z^2 = 1 \}$. I always hear people say that this is closed and that it has no boundary. But isn't every point on the sphere a boundary point? Since for every point $x$ on the sphere, any open ball in $\mathbb{R}^3$ (defined by $\{ y : |...

geometry general-topology
I have asked also in Geometry and Topology chat room: chat.stackexchange.com/transcript/20807/2017/3/15
 

« first day (1737 days earlier)      last day (2604 days later) »