« Section 53: Problem 5 Solution

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 .