- #1
mtayab1994
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Homework Statement
Let A and B be two sets.
Homework Equations
Prove that there exists a injection from A to B if and only if there exists a surjection from B to A
The Attempt at a Solution
I did one implication which is we suppose that f: B→A is a surjection.
Then by definition of a surjection: [tex]\forall a\in A,\exists b\in B/f(b)=a[/tex]
Then for each a in A we consider the nonempty set [tex]f^{-1}(a)[/tex]
and I let an arbitrary b in [tex]f^{-1}(a)[/tex] and then I defined the function:
g: A→B so we get g(a)=b for our choice of b in [tex]f^{-1}(a)[/tex] and this is injective, because f was a function that we established earlier so if we take b1 in [tex]f^{-1}(a_{1})[/tex]
and b2 in [tex]f^{-1}(a_{2})[/tex] then it's simple to see that b1≠b2, but if they were equal then by definition:
a1=f(b1)=f(b2)=a2. So therefore g is injective. Is this correct so far?