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Section 25*: Problem 2 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
(a) Since is path connected, in the product topology is path connected and connected (§24, exercise 8(a)), therefore, there is one component = one path component, i.e. the whole space.(b) preserves the metric, and therefore, by exercise 2 of §21, it is a homeomorphism. Hence, and are in the same component iff and are in the same component. We show that it is iff is bounded.First, we show that is path connected to any other point where . Indeed, is a path. To see this, recall that a function to the subspace is continuous iff it is continuous as a function in to the space, and that the topology in a subspace is generated by the same metric as the topology of the space. Moreover, iff iff . Therefore, the preimage is open.Now, two things follow. First, the space is locally path connected. Therefore, two points are connected iff they are path connected. Second, and are path connected iff is bounded. Indeed, according to the exercise 8 of §23, the sets of bounded and unbounded sequences is a separation, therefore, any point connected to (which is bounded) must be bounded. The other direction: if a sequence is bounded, then the argument similar to one in the previous paragraph shows that there is a path.(c) If then is a path from to . Indeed, there is a finite number of nonzero coordinates in , and, therefore, the preimage of any open basis set of the box topology is a finite intersection of intervals open in . On the other hand, if then there is an infinite sequence of indexes such that and the homeomorphism if or otherwise (it is a homeomorphism by exercise 8 of §19), maps to and to an unbounded sequence. But the sets of bounded and unbounded sequences are open in the box topology (they are open even in the coarser uniform topology) and form a separation. Therefore, and must be in different components. BTW, the result also implies that in the box topology is not locally path connected, still components and path components are the same.