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- March 1st 2010, 10:05 AM #1

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- March 1st 2010, 10:09 AM #2

- March 1st 2010, 10:20 AM #3

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Here is definition which I used:

"Let X be a topological space and , the ring of continuous function on X . The level set of f is defined at is the set . The zero set of f is defined to be the level set of f at 0 . The zero set of f is denoted by Z(f) . A subset A of X is called a zero set of X if A=Z(f) for some .

- March 1st 2010, 10:47 AM #4

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- March 1st 2010, 11:09 AM #5

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- March 1st 2010, 12:38 PM #6
How about this. If is continuous (where is a metric space) we define the zero set . Why is closed? Because, is closed in under the usual topology and since is continuous so is

Alternatively, let . If we're done, so assume not. Then, is a limit point of and so there exists a sequence of points such that . But, since is continuous we know that and so . It follows that which finishes the argument.

What about which is the identity function. is closed in the domain but it's not the zero set.