« Section 16: Problem 9 Solution

Section 16: Problem 10 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 . Compare the product topology on , the dictionary order topology on , and the topology inherits as a subspace of in the dictionary order topology.
The first two topologies are not comparable. Indeed, open in the first topology contains the point , but does not contain any its open neighborhood in the second topology, and open in the second topology contains the point , but does not contain any its open neighborhood in the first topology.
The third topology is strictly finer than the first and second one. Indeed, according to Exercise 9, the third topology is generated by the basis consisting of the sets . So, every basis element of the first topology is the union of some basis sets of the third topology, and every basis set of the second topology is the union of some basis sets of the third topology as well. The fact that the third topology is strictly finer than the first and second topologies follows from the fact that the first and second topologies are not comparable.