# Questions on locally compact space

1. Oct 31, 2012

### jtceleron

Q1. if A is a subset of X, choose the topology on X as {∅,U|for every U in X that A is a subset of U}. Then is this topology a locally compact space?
Q2. X=[-1,1], the topology on X is {∅, X, [-1, b), (a,b), (a,1] | for all a<0<b}. How to prove every open set (a,b) in X is NOT locally compact?

for Q1, there is so few restrictions on X, I don't know whether it's a locally compact space or not, however, I also cannot find a counterexample.
for Q2, when defined open set of a specific topology, does is mean that we have also defined closed set? If so, then for every point a in X, a closed set including a is a compact subset, then it is locally compact, a contradiction.

2. Nov 1, 2012

### Bacle2

Usually, you find a condition satisfied by your space that is not satisfied by locally-compact spaces.

Example : Rationals as a subspace of Reals are not locally compact; in metric spaces,

compactness is equivalent to every sequence having a convergent subsequence.

Then, e.g. the sequences given by:

1) a1=1, a1=1.4 , a3=1.41 ,.... (first n terms of the decimal expansion of √2 )

2) a1=1 , a2=2,..., an =n ,....

are sequences without convergent subsequences. In non-metric spaces it is a little harder.

Still, re #2 : I have never seen a description of local compactness in terms of open sets,

but in terms of points. What definition are you using?

3. Nov 1, 2012

### jtceleron

I mean, in #2, that if every point in the open set (a,b) has a compact subset, then the open set is locally compact.

4. Nov 1, 2012

### jtceleron

in #1. the topology on X has already defined, which mean if choosing rationals as the subset A, then U are all the subsets including A, it is like a discrete topology, so imposing a metric topology on it seems not right.