Is There a Bijection from A to X if B is a Subset of X?

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In summary, the conversation discusses the possibility of there existing a bijection from a set A to a subset X of set B, where A and B are bijective. The conclusion is that this is only possible for infinite sets if X has the same cardinality as B. Examples using the sets of natural numbers and subsets of odds and evens are suggested to further illustrate this concept.
  • #1
aaaa202
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This is something I have been wondering about. Let f:A->B
be a bijection. If B is a subset of X. Can there still exist a bijection from A to X?
 
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  • #2
But A is already fully mapped onto B, so which elements of A are left to map onto the X - B?
 
  • #3
Well the cardinality of the rationals, natural numbers and even natural numbers are the same.
So there exists a bijection between each of these sets even though, the even natural numbers are a subset of the natural numbers. So I think you're actually wrong?
 
  • #4
I asked a question; I did not provide an answer.

However it seems that you now know the answer: I think you are saying "for finite sets the answer is NO; for infinite sets the answer is YES". Next you should go ahead and construct an example using the set of natural numbers and subsets of odds and evens.
 
  • #5
You might want to re-write the question to specify that B is a proper subset of X.
 
  • #6
UltrafastPED said:
I asked a question; I did not provide an answer.

However it seems that you now know the answer: I think you are saying "for finite sets the answer is NO; for infinite sets the answer is YES". Next you should go ahead and construct an example using the set of natural numbers and subsets of odds and evens.
It should be mentioned that it isn't always true for infinite sets. E.g., take ##A = B = \mathbb{R}## and ##X = \mathbb{Z}##. In general it will be possible only if ##X## has the same cardinality as ##B## (and hence the same cardinality as ##A##).
 

Related to Is There a Bijection from A to X if B is a Subset of X?

1. What is a bijection?

A bijection is a type of function in mathematics where every element in the domain (A) is paired with a unique element in the codomain (B). This means that every element in A has one and only one corresponding element in B, and vice versa.

2. How do you know if a function is a bijection?

To determine if a function is a bijection, you must check two things: injectivity and surjectivity. A function is injective if each element in the codomain (B) has at most one corresponding element in the domain (A). A function is surjective if every element in the codomain (B) has at least one corresponding element in the domain (A). If a function satisfies both of these conditions, it is a bijection.

3. What is the significance of a bijection in mathematics?

Bijections are important in mathematics because they provide a one-to-one correspondence between two sets. This allows for the manipulation and comparison of elements in the two sets, making it easier to solve problems and prove theorems.

4. Can a function be a bijection if the domain and codomain are not equal?

Yes, a function can still be a bijection even if the domain and codomain are not equal. As long as every element in the domain has a unique corresponding element in the codomain, and vice versa, the function is considered a bijection.

5. What is the inverse of a bijection?

The inverse of a bijection is a function that reverses the mapping of the original bijection. This means that the domain and codomain switch places, and the direction of the mapping is reversed. The inverse of a bijection is also a bijection, as every element in the new domain has a unique corresponding element in the new codomain, and vice versa.

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