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Empty set as a subset?

  1. Aug 28, 2012 #1
    Ok im a bit confused here. According to the definition of a proper subset means that everything in set A is in set b and a set always contains an extra pair of brackets. But in this example
    C={∅,{∅}} why is this correct ∅ ⊆C instead of {∅} ⊆C for the first object?
     
  2. jcsd
  3. Aug 28, 2012 #2
  4. Aug 28, 2012 #3
    I'm a bit confused as to what you're asking. What do you mean extra brackets? A proper subset just means that if A is a proper subset of B, A is a subset of B and A =/= B.

    If you're just asking why the empty set is always a subset, just look at what it would mean if it weren't. If the empty set weren't a subset of A, then that would mean the empty set contains some element that is not in A. But, that's impossible because the empty set has no elements.

    Steve - right click the number and copy the link address, or just look at the format here:
    https://www.physicsforums.com/showpost.php?p=4048220&postcount=5
     
  5. Aug 28, 2012 #4
  6. Aug 28, 2012 #5
    im asking is that if i say that ∅⊆C is this correct?
     
  7. Aug 28, 2012 #6
    Sure is.

    Feel free to read that symbol as "Is a subset of OR is equal to." Since the empty set is a subset of anything, that statement is true and tautologous for any arbitrary C.
     
  8. Aug 28, 2012 #7
    does this mean that what i said means that it points to the first empty set element within the set C?
     
  9. Aug 28, 2012 #8
    What???
     
  10. Aug 28, 2012 #9
    how would i say that the first empty set in set C is a subset of C?
     
  11. Aug 28, 2012 #10
    With what you just wrote.

    ∅ is the empty set.
    {∅} is.. "the set of the empty set."
     
  12. Aug 28, 2012 #11
    oh ok. I was just a bit confused since you usually you put brackets around something when your saying that a set is a subset of another set such as this-

    C={4,5,6) D={1,2,3,4,5,6}

    {4,5,6}⊆{1,2,3,4,5,6}

    But with the empty set I assume don't need to put brackets around it unless its a set within another set
     
  13. Aug 29, 2012 #12

    HallsofIvy

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    Specifically, {∅} is NOT empty- it contains one element, the empty set. The set you give, C= {∅,{∅}} contains two elements, the empty set and the set whose only member is the empty set. Here it is perfectly correct to say that ∅[itex]\subset[/itex] C (the empty set is a subset of any set, as you say), {∅}[itex]\subset[/itex] C because ∅ is a member of C, and {{∅}}[itex]\subset[/itex] C because {∅} is a member of C.;
     
  14. Aug 31, 2012 #13
    oh ok i get it but how about this one A={4,{5},6} and B={{5},6,7}. would A be a proper subset of B?
     
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