# Solutions to Chapter 1: Topology, Hocking and Young

Exercise 1-4: Prove that the collection of all open half planes is a subbasis for the Euclidean topology of the plane.

Proof 1-4:

Note: In the Euclidean plane, we can take as a basis the collection of all interiors of squares.

Note also that a subcollection $\mathcal{B}$ of all open sets of a topological space S is a SUBBASIS of S provided that the collection of all finite intersections of elements of $\mathcal{B}$ is a basis for S.

Clearly, from all open half planes ( $x > x_{i}$ and $y > y_{i}$), we can create a collection of all interior of squares.

Hence, the collection of all finite intersections of all open-half planes satisfies:

Axiom $\mathcal{O_{2}}$: the intersection of a finite intersection of open half planes is an open set. (interior of a square).

Axiom $\mathcal{O_{1}}$; (also). The union of any number of finite intersections of all open half planes is also open set (interior of a square).

Axiom $O_{3}$: (clearly). $\phi$ and S are open.

QED.

Exercise 1-5:

Let S be any infinite set. Show that requiring every infinite subset of S to be open imposes the discrete topology on S.

Proof 1-5:

Case: S is countable. We neglect the subcase that a selected subcase is finite. (I am using Prof. Rudin’s definition of countable). The other subcase is that there exists a subset $X_{1} \subset S$, where $X_{1}$ is countable. Let $X_{1}$ be open. Hence, $S-X_{1}$ is closed. But $S-X_{1}$ is also countable. Hence, $S-X_{1}$ is also open. Hence, there is no limit point. Hence, the topology is discrete, that is, there is no limit point.

Case: S is uncountable. Consider again a proper subset $X_{1} \subset S$; hence, $X_{1}$ is open by imposition of hypothesis. Hence, $S-X_{1}$ is closed. But, $S-X_{1} \neq \phi$ and not finite also. Hence, $S-X_{1}$ is infinite. Hence, $S-X_{1}$ is open. Hence, there are no limit points. Hence, it is a discrete topology in this case also.

QED.