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Help with a logical derivation of set theoretical statement

  1. Feb 11, 2011 #1
    1. The problem statement, all variables and given/known data
    This is actually from the proof of Dedekind's cut in Rudin's Principles of Mathematical analysis on the page 19. It says when [tex]\alpha\in\mathbb{R}[/tex] ([tex]\alpha[/tex] is a cut) is fixed, [tex]\beta[/tex] is the set of all [tex]p[/tex] with the following property:

    There exists [tex]r>0[/tex] such that [tex]-p-r\notin\alpha[/tex].​

    From the given above, I need to derive that

    if [tex]q\in\alpha[/tex], then [tex]q\notin\beta[/tex].​

    But I cannot reach this statement as my explanation for this is in the below.

    2. Relevant equations

    3. The attempt at a solution
    The draft I have done so far is that, as defining [tex]\beta[/tex] such that

    [tex]\beta=\left\{p|\exists r\in\mathbb{Q} (r>0 \wedge -p-r \notin \alpha)\right\}[/tex],​

    I derived

    [tex]p \notin \beta \leftrightarrow
    \forall r \in \mathbb{Q} (r>0 \to -p-r\in\alpha)[/tex].​

    And I'm stucked here. From the last statement, I cannot derive the conclusion I was meant to derive. If anyone gives me help, I will give thanks.
    Last edited: Feb 12, 2011
  2. jcsd
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