Exploring the Cardinality of a Funny Bag

In summary, the conversation discusses the concept of a bag and how it can be represented using sets of ordered pairs. The question of the cardinality of this type of bag is raised, and there is a discussion about whether or not it can be constructed in certain set theories. Ultimately, it is determined that it is a proper class.
  • #1
dodo
697
2
Hello, I have an aficionado curiosity, so please bear with me.

As you know, bags are sets where repeated elements are allowed. Imagine the following funny representation for a bag: instead of repeating elements, we use a set of ordered pairs, containing each distinct item plus a count of how many of its kind there are. (Suppose, for the sake of the argument, that the number of repetitions is countable.) Now the funny part: we can store items which are not in the bag by using a count of zero. The question is, what is the cardinality of such a monster?

In order to represent, say, the integers, you save each of them with a count of one. But then you also save the (rest of the) reals with a count of zero, so the cardinality of this set is at least aleph-1; but you also save elements with a zero count from sets of size aleph-2, aleph-3...

You might argue that the construction is paradoxical by design, since I am simply asking for a set which is bigger than anything you can construct. But I suspect such rejection would have an interesting consequence.

Take, from the example above, the subset of the zero-count elements, from sets of size aleph-1, -2, -3... The individual items are, in themselves, sets (Cauchy series), whose elements have, in turn, being taken from the "previous smaller set": the reals being series of elements from a set of cardinality aleph-0, and so on. So this subset is also a subset of a potentially bigger one, "the sets which do not contain themselves as a member", a phrase which should ring a bell. Would we be saying that the Russell Paradox does not exist, since it's based on a premise which is flawed in the first place?
 
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  • #2
The axiom (schema) of specification won't allow you to construct this multiset. In ZF, it doesn't exist at all. In NBG, it's a proper class (I believe).
 
  • #3
Yes, in NBG it would definitely be proper: it has an evident surjection onto the class of all things.

At least, it does if he means what it sounds like he means. e.g. that he means to represent the empty bag as
{ (x, 0) | x is a thing }​
 
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  • #4
I don't know much about set theory, but since we usually consider
{a, a, a, b, b, c, a, b, c, c, b} and {a, b, c} as the same set I'd say we just agree that
{ (n, 1) | n an integer} [itex]\cup[/itex] { (r, 0) | r a real non-integer number } and { (n, 1) | n an integer } the same set (throwing out all elements with count 0).

But perhaps this is just too naïve.
 
  • #5
Hurkyl said:
Yes, in NBG it would definitely be proper: it has an evident surjection onto the class of all things.

It's clearly not a set. I wasn't sure if it could be constructed at all in NBG -- but I thought that it could be and that as such it would be a proper class. But you're right, limitation of size does mean that it exists.
 

What is the definition of cardinality?

Cardinality refers to the number of elements in a set or collection. It measures the size or quantity of the set.

How is cardinality different from size?

While cardinality and size both refer to the number of elements in a set, they are calculated differently. Cardinality takes into account the uniqueness of the elements, while size does not.

What is the importance of exploring the cardinality of a funny bag?

Exploring the cardinality of a funny bag can provide insight into the variety and uniqueness of the items within the bag. It can also help in understanding the distribution of different types of items within the bag.

What factors can affect the cardinality of a funny bag?

The cardinality of a funny bag can be affected by the size of the bag, the types of items inside, and the randomness of the selection process.

How can cardinality be calculated for a funny bag?

To calculate the cardinality of a funny bag, first identify the unique elements within the bag. Then, count the number of each unique element and sum them up to get the total cardinality.

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