- #1
h.shin
- 7
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Homework Statement
If A and B are sets, prove that a subset [itex]\Gamma\subset A X B[/itex] is the graph of some function from A to B if and only if the first projection [itex]\rho: \Gamma\rightarrow A[/itex] is a bijection.
Homework Equations
The Attempt at a Solution
I first thought that i should define bijection by saying that a a bijection exists when there is both injection and surjection.
If [itex]\rho: \Gamma\rightarrow A[/itex] is not a bijection then it is either
1)not surjective
2)not injective
3)both 1) and 2)
So,
I thought that i should prove that [itex]\Gamma[/itex] is not the graph of some function A -> B when the first projection is not bijective by showing the non-surjective and non-injective cases separately. And then prove that it is, in fact, [itex]\Gamma[/itex] is the graph of the function when the first projection is bijective.
So, i have to show the cases for when the first projection is not injective, when it's not surjective, and then when it is bijective.
But, how do i prove that the [itex]\Gamma[/itex] is either the graph of the function or not? in any of the 3 cases?
I just don't know where to start. Is this the right approach or is there a shorter way to do this?