Given a family of topologies on a fixed set the smallest topology in which any -open set is open is called the least upper bound topology. It is a standard fact that when each satisfies a separation axiom , then the supremum topology is also for . However, this cannot be strengthened to . I could not find any explicit examples on the web or in literature, so I constructed some and described them here.

# Definitions

Before we go to the details, let us fix the notation.

Def. *Least upper bound topology*. Given a family of
topologies
on a given set
the least upper bound topology
is generated by the subbase
Sometimes we abuse notation and denote the supremum
topology by

Def. Topological space is when it is and normal: for two non-trivial, disjoint closed sets there are disjoint open sets that separate them.

Def. *Sorgenfrey line.* Equip set of reals with the topology
generated by
. The resulting topology,
, is called the Sorgenfrey line. Note that
is
-open. Hence, the
is strictly finer (larger) than the topology on Euclidean
line.

# Sketch of construction

We will construct two spaces such that the least uper bound topology is not Now I begin to describe the construction, so this is the last call to try to find one for yourself!

We will first argue that the “half Euclidean, half Sorgenfrey plane”, that is, the product of the real line with the Euclidean topology, , and the Sorgenfrey line, is normal.

Then we will show that the least upper bound topology of and is just a Sorgenfrey plane — a classic example of a non-normal space (to see this play with, spoiler alert, the set ).

Then I will show a bonus example that generalizes.

# The construction

The Sorgenfrey line is not only , but also

Def. Space is when for any two closed, non-trivial disjoint sets there is a continuous function such that and

To show that the Sorgenfrey line is we will use the following characterization of spaces.

(Vedenissoff) Given a topological space the following are equivalent:

is .

Every closed set can be written as a countable intersection of open sets (= set).

The Sorgenfrey line is because for any closed set we have

Now, knowing that the Sorgenfrey line is , we can use the next theorem to show that the product of the Sorgenfrey line with the Euclidean line is normal.

A product of a space and a metric space is

For details see Dan Ma’s post.

Summing up: Both and are (the order in which the product is taken does not matter).

It remains to show that the supremum topology of these two topologies, is the same as the topology on the Sorgenfrey plane, , which is not .

First note that a basic open set of a Sorgenfrey plane, that is, a rectangle , is open in the upper bound topology because we can write it as an intersection of basic open sets:

Now the other way around. Let be any subbasic set from the supremum topology. By definition, is open in at least one of two topologies or . Without loss of generality, we assume that is open in . Hence, there is an index set such that But so each is also -open. Hence, is just a union of a product -open sets. But this is a definition of an open set in the product topology. Therefore, is open in the Sorgenfrey plane. □

# Bonus example

More elementary construction (but using an infinite family of ’s) of non- supremum topology made from spaces is this: It is known that is not normal ( with the discrete topology). We argue that the following supremum topology is not : where consists of the empty set and sets Each is because it is homeomorphic to the discrete space on natural numbers. On the other hand, is homeomorphic to . □

More generally, since every
space is embeddable in the product of
, I think, we could make an analogous construction as above
and represent *any*
space as the upper bond topology of a product of
spaces.

# Notes

It is easy to create a family of spaces that are normal (closed sets can be separated by open sets, but space might not be T1), but whose supremum topology is not normal. For example, let . Define three topologies Each of these topologies is vacuously normal, but is not normal.

I think an appropriate way to finish this post is by linking to *This is not
normal* by Negativland.