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Section 32: Problem 9 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
We show that , where is uncountable, is not normal.(a) is a finite subset of , , . Every is open ( is in the discrete topology). If , is open in , then there is a finite set of indexes such that implies . Therefore, and they form a basis.(b) Let , i.e. may have many coordinates equal to but all other coordinates different. are closed and disjoint. Indeed, if then and does not intersect . For let . Since is injective on , is countable. Therefore, there is uncountable set of indexes such that for , i.e. cannot be a point in any other .Suppose and separate and and derive a contradiction.(c) Essentially, what we do here: we want to construct a sequence of points in such that they "converge" eventually to a point of the form: 1 for all indexes except a countable subset of indexes for which it takes values 1, 2, 3, .... We take the first point equal to 1, then for the first indexes in a countable sequence of indexes we define values from 1 to ( ), then do the same for the first indexes ( ), etc. Moreover, we want to define the sequence of indexes and the sequence of "steps" such that . Note that takes values different from 1 on the set while includes the next step as well. Ok, so, for all and . Given and we first define such that (this is possible because and we can define ) and then just define by the expression.(d) Now, given the sequence of points and the sequence of points , we define a point in equal to 2 for all points not in and for . , so there is a finite set such that . Since is finite, there is such that . Let for , for and for all other indexes. Then for all either and or and . For all either and or and . Therefore, .