Section 32: Problem 9 Solution
Working problems is a crucial part of learning mathematics. No one can learn topology merely by poring over the definitions, theorems, and examples that are worked out in the text. One must work part of it out for oneself. To provide that opportunity is the purpose of the exercises.
James R. Munkres
We show that
, where
is uncountable, is not normal.(a)
is a finite subset of
,
,
. Every
is open (
is in the discrete topology). If
,
is open in
, then there is a finite set
of indexes such that
implies
. Therefore,
and they form a basis.(b) Let
, i.e.
may have many coordinates equal to
but all other coordinates different.
are closed and disjoint. Indeed, if
then
and
does not intersect
. For
let
. Since
is injective on
,
is countable. Therefore, there is uncountable set of indexes
such that
for
, i.e.
cannot be a point in any other
.Suppose
and
separate
and
and derive a contradiction.(c) Essentially, what we do here: we want to construct a sequence of points in
such that they "converge" eventually to a point of the form: 1 for all indexes except a countable subset of indexes for which it takes values 1, 2, 3, .... We take the first point
equal to 1, then for the first
indexes in a countable sequence
of indexes we define values from 1 to
(
), then do the same for the first
indexes (
), etc. Moreover, we want to define the sequence
of indexes and the sequence
of "steps" such that
. Note that
takes values different from 1 on the set
while
includes the next step as well. Ok, so,
for all
and
. Given
and
we first define
such that
(this is possible because
and we can define
) and then just define
by the expression.(d) Now, given the sequence of points
and the sequence of points
, we define a point
in
equal to 2 for all points not in
and
for
.
, so there is a finite set
such that
. Since
is finite, there is
such that
. Let
for
,
for
and
for all other indexes. Then for all
either
and
or
and
. For all
either
and
or
and
. Therefore,
.