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Discrete Mathematics - Void Sets being Subsets of other Void Sets

  1. Aug 27, 2012 #1
    1. The problem statement, all variables and given/known data

    Hello.

    Here is the question:
    Determine whether or not R is some sort of order relation on the given set X.

    X = {∅, {∅}, {{∅}} } and R ε ⊆.

    I can't seem to figure out why the ordered pairs given are what they are.

    2. Relevant equations

    None.

    3. The attempt at a solution

    What I first wrote out was:
    R = { (∅, {∅}), ({∅}, {{∅}}), (∅, {{∅}}) }

    Which is missing some ordered pairs. Also, my book says ({∅}, {{∅}}) is not an element of ⊆.

    I tried to use my limited logic to understand the answer given, and this is what I got:

    (∅, ∅) is an ordered pair because all the elements of ∅ are in ∅.
    (∅, {∅}) is an ordered pair because ∅ is a member of {∅}.
    (∅, {{∅}}) This causes some confusion. My book says the only member of {{∅}} is {∅}, but the first coordinate has to be a subset of the second coordinate. If ∅ is not an element of {{∅}}, how can it be a subset?
    ({∅}, {∅}) is an ordered pair because they are equal and subsets of each other.
    ({{∅}}, {{∅}}) same as above.

    I don't understand why {∅} is not a subset of {{∅}}. {∅} is an element of {{∅}}, and should be a subset of it to my understanding because all the elements of {∅} are also within {{∅}}.
     
  2. jcsd
  3. Aug 27, 2012 #2

    HallsofIvy

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    Staff Emeritus
    Science Advisor

    I think you are not distingishing between being a subset and being a member of a set.
    {∅} is not a subset of {{∅}} because {∅} contains the empty set as a member and {{∅}} does NOT. "All the elements of {∅} are also within {{∅}}" is not true. The only member of {∅} is the empty set and that is NOT a member of {{∅}} because it only member is {∅}, not the empty set. The empty set is a subset of every set but not necessarily a member.
     
  4. Aug 27, 2012 #3
    Thanks for the reply HallsofIvy. That explains why ∅ is a subset of {{∅}}, while {∅} is not. ∅ is a subset of every set, while {∅} is not because that it is the set that contains only ∅. Is that correct?
     
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