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Homework Help: Proof about onto functions.

  1. Dec 24, 2012 #1
    1. The problem statement, all variables and given/known data
    Given any set A, there does not exist a function f:A→P(A)
    It wants us to do a proof by contradiction and assume there is an onto function.
    For each element [itex] a \in A [/itex] f(a) is a particular subset
    of A. The assumption that f is onto means that every subset of A appears as f(a)
    for some [itex] a \in A [/itex] To arrive at a contradiction, we will produce
    a subset [itex] B \subseteq A [/itex] that is not equal to f(a) for
    any [itex] a \in A [/itex]
    [itex] B=\{a\in A:a\notin f(a)\} [/itex]
    If f is onto then it must be the case that B=f(a') for some a' in A.
    the contradiction arises when we consider if a' is an element of B.
    First show that the case that a' is in B leads to a contradiction.
    Now finish the argument by showing that the case a' is not in B
    is equally unacceptable.
    3. The attempt at a solution
    If a' is in B then a' is in A
    and if f is onto then f(a') is in B
    if f(a') is in B then f(a') is some a
    in A but this is a contradiction
    because we assumed that a was not in f(a).
    If a' is not in B then it is not in A and therefore
    it is not in anything.
    Am I headed the right direction?
    Last edited: Dec 24, 2012
  2. jcsd
  3. Dec 24, 2012 #2


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    Homework Helper

    This is somewhat confused.

    Given [itex]f[/itex], [itex]B[/itex] consists of exactly those elements [itex]a \in A[/itex] which are not members of their image, [itex]f(a) \subset A[/itex]. However there could be other elements of A which are members of their image.

    The point is that if [itex]f[/itex] is onto, then by definition [itex]B \subset A[/itex] must be the image of some [itex]a' \in A[/itex], so that [itex]f(a') = B[/itex]. Note that, by assumption, [itex]a' \in A[/itex]. Whether [itex]a' \in B[/itex] is another question.

    So what happens if [itex]a' \in B[/itex]? That would mean that [itex]a'[/itex] is not a member of its image. But its image is B. This is a contradiction.

    Now you need to show that a similar contradiction arises if you assume [itex]a' \notin B[/itex].
  4. Dec 24, 2012 #3
    is image f(a)=a
  5. Dec 24, 2012 #4


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    Homework Helper
    Gold Member

    a' was chosen to be in A.
    a' was chosen such that f(a')=B. It cannot be "in B".
    a is an element of A; f(a) is a subset of A. They cannot be equal.
    Start again at "If a' is in B then " and use the definition of B to deduce something about a'.
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