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Homework Help: Advanced calc./tricky?

  1. Dec 12, 2007 #1
    1. If set A is compact, show that f(A) is compact. Is the converse true?

    Ans. f:M to N is continuous and A subset M is compact. Then f(A) is compact.
    The converse is not necessarilly true. For Ex: F(x)=0 for every x in R(real #'s) and
    k={0}. Then f^-1(k)=R is not compact.

    2. If set A is connected, show that f(A) is connected. Is the converse true?

    Ans. f:M to N is continuous and A subset M is connected. Then f(A) is connected.
    The converse is not necessarilly true. For Ex: F(x)=x^2 and k=1.
    Then f^-1(k)={-1,1} which is not connected.

    3. If set B is closed, show that B inverse is closed.

    Ans. f is continuous on B if f is continuous on every x sub 0 element B.
    a. f is continuous on B
    b. for every x sub n to x sub 0 in A. f(x sub n) approaches f(x sub 0)
    c. for any u open in N, f^-1(u) is open in M
    d. for any F closed in N, f^-1(F) is closed in M.

    If anyone can add to this I would be greatfull.
  2. jcsd
  3. Dec 13, 2007 #2
    I would say that your solutions for 1 and 2 really don't seem to be sufficient, unless you're already told that the continuous images of compact/connected sets are compact/connected, in which case this is a trivial question. Is f continuous? For these to be true it must be but you haven't stated that anywhere. Furthermore, this constitutes a topology question rather than an analysis question.

    1) To show that a set is compact it is necessary to show that every family of covers for that set has a finite subcover. In regard to this question, we need to show that if C is a cover for f(A), then there exists a finite subcover of f(A).

    Let C be a cover of f(A), and define

    [tex] C^\prime = \{ f^{-1}(U) | U \in C \} [/tex]

    Now what can we say about each [itex] U \in A[/itex] given that f is continuous?

    Furthermore, what is [itex] C^\prime [/itex] with respect to A? How can we use the relation between [itex] C^\prime [/itex] and A to find a finite subcover of the image?

    2) For the sake of contradiction, assume that f(A) is not-connected. That is, it can be written as the disjoint union of two open sets. Since f is continuous, consider the pre-image of these sets. Why does this give a contradiction?
  4. Dec 13, 2007 #3


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    Open covers, that is!
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