Compactification and Extension of maps.

  • Thread starter WWGD
  • Start date


Science Advisor
Gold Member
Hi: a couple of questions on (Alexandroff) 1-pt. compactification:

Thanks to everyone for the help, and for putting up with my ASCII posting
until I learn Latex (in the summer, hopefully.)

I wonder if anyone still does any pointset topology. I see many people's
eyes glace when I talk about, e.g., normal spaces. Anyway:

1) I am trying to show that if I have a compact space X and remove an
arbitrary point x_0 from this space, then the resulting space X-x_0 is
locally compact (where I assume that the definition of compact and locally
compact include Hausdorff)

I think we can say that an open subset of a locally compact space is locally compact.
and then we can use the fact that {x_0} is closed, and then apply this.

But I wonder if this is also a way of doing it:
Maybe one can reverse/invert the process of 1-pt. compactification CX of a compact
Hausdorff space X, since CX is Hausdorff iff X is locally compact, i.e., we
start with a compact, Hausdorff space X, (assumed to be the 1-pt. compactification
of some other space Y ) , remove a point x_0 and then Y must have been Hausdorff
and locally compact to start with.

2) Extending functions f :X-->X to functions f^:CX-->CX , with

CX the compactification of X ( i.e., so that f^|_X=f ,with

f^|_X the restriction of f^ to X ) , so that the extension is continuous

or analytic ( I am thinking of CX as the Riemann Sphere, with X =Complex Plane)

Under what conditions can we do this?. All I (think) I know is that

f:X-->X can be extended continuously if f is regular ( inverse image of every

compact set is compact) . What conditions do we need to extend a

function f that is analytic in the complex plane into a function f^ that is

analytic in the sphere, , i.e., when/how can we find f^: Riemann Sphere to

itself, and f^|_X (restriction to complex plane) =f ?.


Want to reply to this thread?

"Compactification and Extension of maps." You must log in or register to reply here.

Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving