- #1
mmilton
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Homework Statement
Let f: A --> B be an injection and suppose that the set A is countably infinite; how can I prove that there is an injection from B to A if and only if B is countably infinite?
Also, if we would suppose that A is uncountable, can B be countable?
Homework Equations
The Attempt at a Solution
Here is what I have thus far,
First direction:
Suppose B is countably infinite. Then, by definition, there is a
bijection from B to the naturals. Since A is also countably infinite, there is a bijection
from A to the naturals, and hence a bijection between B and A (and hence injection from B to A).
Next direction:
First show that since f is an injection of a countably infinite set to B, then B must be infinite.
Now, if there is an injection from B to A, then there is a bijection from B to a subset of A, call this subset S. So B and S have the same cardinality. But any infinite subset S of a countably infinite set A is countably infinite, so B has the same cardinality as a countably infinite set S.