Section 53: Problem 6 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
Let
be a covering map.
(a) If
is Hausdorff, regular, completely regular, or locally compact Hausdorff, then so is
. [Hint: If
is a partition of
into slices, and
is a closed set of
such that
, then
is a closed set of
.]
(b) If
is compact and
is finite for each
, then
is compact.
Regarding the hint:
is closed in
, so that it has no limit points outside of
, but
is closed in
. The hint immediately implies that if
is a
-space then so is
.
(a) If
is Hausdorff, take two points
, and if
, then
has a neighborhood
evenly covered by
so that
and
are in different open sets of
homeomorophic to
, but if
then
and
have disjoint open neighborhoods and their preimages are disjoint open neighborhoods of
and
.
If
is regular (locally compact Hausdorff), then using the fact that a
-space (Hausdorff space) is regular (locally compact) iff for every point
and its open neighborhood
there is an open neighborhood
of
such that
(is compact), see Lemma 31.1(a) (Theorem 29.2), and knowing already that
is a
-space (Hausdorff space), if
where
is open (
is open), we take a neighborhood
of
evenly covered by
so that its preimage
, some
such that
, and, finally, a neighborhood
such that
(is compact), to have
, a homeomorphism between
and
, and
, an open neighborhood of
such that
(
is a homeomorphism) is a closed (compact) subset of
, and of
(using hint).
If
is completely regular, for every point
and its open neighborhood
, if
and its open neighborhood
is evenly covered by
(
,
), there is
such that
and
(where
, and hence,
, is open as
is open), and
such that
if
and
otherwise is continuous on both closed sets
(where it equals
) and
(where it equals
), hence, on
, and is as required.
(b) If
covers
, for a point
choose, first,
evenly covered by
,
, and then
where
for some
such that
, noting that
is then an open neighborhood of
evenly covered by
, then take a finite subcover of
,
, and consider
such that if
and
, then
.