Constructing and Recognizing Open and Closed Sets
Propositions 5.14 and Corollary 5.15 are very useful tools for
constructing open and closed sets. The reason is that most of the sets
that one works with in mathematics are specified in the following form:
On the other hand, the preimage of a set S under a function f is of the form:
So, the basic principle to constructing open or closed sets or to recognizing that a given set is open or closed, is to rewrite the condition:
Of course the trick is finding the right function and the right open or closed set. The next few examples illustrate what's going on:
It remains to show that the function f(x,y)=y-mx-c is continuous, and that the set S is open. Of course we know that open intervals are open in the real line so S is certainly open. To see that f is continuous we use the sort of arguments that we developed in the section: Convergence and continuity in Euclidean spaces.
It was very easy to see that the set S was open. This is because the open intervals in the real line provide us with a plentiful supply of sets we know to be open. For this reason we usually aim to have the functions we construct for this type of argument always map into the real numbers.
Likewise, we know that closed intervals in the real line are closed sets so when we want to show a set is closed we usually try to get a function which maps into a closed interval in the real line.
Question: Why is S closed?
The set S of all real numbers less than 1 is open, because it is an open interval.
Question: Why is f continuous?