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Section The Quotient Topology

Given a topological space X and a surjection p from X to a set Y, we can use the topology on X to define a topology on Y. This topology on Y identifies points in X through the function p. The resulting function on Y is called a quotient topology. The quotient topology gives us a way of creating a topological space which models gluing and collapsing parts of a topological space.

Preview Activity 16.1.

Let X={1,2,3,4,5,6} and let Ο„={βˆ…,{1,2},{4,6},{1,2,4,6},X}. Let Y={a,b,c,d} and define p:Xβ†’Y by
p(1)=b, p(2)=a, p(3)=c, p(4)=d, p(5)=c,  and  p(6)=a.
Our goal in this activity is to define a topology on Y that is related to the topology on X via p.

(a)

We know the sets in X that are open. So let us consider the sets U in Y such that pβˆ’1(U) is open in X. Define Οƒ to be this set. That is
Οƒ={UβŠ†Y∣pβˆ’1(U)βˆˆΟ„}.
Find all of the sets in Οƒ.

(d)

Show that Οƒ is the largest topology on Y for which p is continuous. That is, if Οƒβ€² is a topology on Y with Οƒβ«‹Οƒβ€², then p:(X,Ο„)β†’(Y,Οƒβ€²) is not continuous.