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.