Separation Properties
There’s a whole list of properties of topological spaces that we may want to refer to called the separation axioms. Even when two points are distinct elements of the underlying set of a topological space, we may not be able to tell them apart with topological techniques. Points are separated if we can tell them apart in some way using the topology. Today we’ll discuss various properties of separation, and tomorrow we’ll list some of the more useful separation axioms we can ask that a space satisfy.
First, and weakest, we say that points and
in a topological space
are “topologically distinguishable” if they don’t have the same collection of neighborhoods — if
. Now maybe one of the collections of neighborhoods strictly contains the other:
. In this case, every neighborhood of
is a neighborhood of
. a forteriori it contains a neighborhood of
, and thus contains
itself. Thus the point
is in the closure of the set
. This is really close. The points are topologically distinguishable, but still a bit too close for comfort. So we define points to be “separated” if each has a neighborhood the other one doesn’t, or equivalently if neither is in the closure of the other. We can extend this to subsets larger than just points. We say that two subsets
and
are separated if neither one touches the closure of the other. That is,
and
.
We can go on and give stronger conditions, saying that two sets are “separated by neighborhoods” if they have disjoint neighborhoods. That is, there are neighborhoods and
of
and
, respectively, and
. Being a neighborhood here means that
contains some open set
which contains
and
contains some open set
which contains
, and so the closure of . We see that the closure of is contained in the open set, and thus in
. Similarly, the closure of
must be contained in
.
is contained in the complement of
, and similarly the closure of
is in the complement of
, so neither
nor
can touch the other’s closure. Stronger still is being “separated by closed neighborhoods”, which asks that
and
be disjoint closed neighborhoods. These keep
and
even further apart, since these neighborhoods themselves can’t touch each other’s closures.
The next step up is that sets be “separated by a function” if there is a continuous function so that for every point
we have
, and for every point
we have
. In this case we can take the closed interval
whose preimage must be a closed neighborhood of
by continuity. Similarly we can take the closed interval
whose preimage is a closed neighborhood of
. Since these preimages can’t touch each other, we have separated
and
by closed neighborhoods. Stronger still is that
and
are “precisely separated by a function”, which adds the requirement that only points from
go to
and only points from
go to
.
This list of separation
“Being a neighborhood here means that U contains some open set which contains A, and so che closure of A is contained in the open set, and thus in U”
I think I’m not getting this…
Okay.. remember how we defined a neighborhood of a point? It doesn’t just contain the point, it contains an open set containing the point. So here, we start with the set
and say that
is a neighborhood of
if there is an open set
with
.
The open set
provides the “wiggle room” we need, and
has at least as much as
does.
The problem is the closure, maybe I’m misreading, but haven’t you written that if A is a set and O (open set) contains A, then O contains the closure of A?
Oh, gr.. I went too fast there and wrote down the wrong line of reasoning. Let me fix that reasoning in the original.
[…] Now that we have some vocabulary about separation properties down we can talk about properties of spaces as a whole, called the separation […]
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