例句與造句

1. I know that a space X is Lindel鰂 if every open cover has a countable subcover and that X is a second countable space of its topology has a countable basis.
2. I also know that the a second countable space is Lindel鰂 but in order for a Lindel鰂 space to be second countable, our space in question must be a metric space.
3. Thus, if one has a countable base for a topology then one has a countable local base at every point, and hence every second countable space is also a first-countable space.
4. It's difficult to find second countable space in a sentence. 用second countable space造句挺難的