What is the proof for showing a space is connected?

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SUMMARY

The discussion centers on the proof that a topological space X is connected if and only if the only subsets of X that are both open and closed are the empty set and X itself. The proof referenced is found on page 14 of a PDF document linked in the discussion. A participant expresses confusion regarding the conditions under which a subset S can be considered part of X, particularly when S is not the empty set or X. The example of the set of real numbers and the subset [0,1] is used to illustrate this confusion.

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Homework Statement


Show that X is connected if and only if the only subsets of X that are both open and closed are the empty set and X.

Proof: https://files.nyu.edu/eo1/public/Book-PDF/Appendix.pdf
Page 14.

I'm confused by this proof. First, if S is not in {null set, X} then how can S be a subset of X?
Secondly, how can X = S U (X\S)?
 
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Suppose that X is the set of real numbers. Then we could let S be the set [0,1]. {null,X} contains only two elements: the null set, and the set of real numbers, neither of which are [0,1]
 
OK thanks. I didn't consider X to be a single element, then any nonempty subset of X would not be in {null, X}
 

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