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QUESTION
Let A be a non-empty set. Prove that if there exits an injection f : A - N(natural numbers), then A is finite or countably infinite.
1. Let A be a non-empty set. Prove that if there exits an injection f : A -> N(natural numbers), then A is finite or countably infinite.
2. Prove that on the real line R, the collection of all closed intervals with rational endpoints is a countable set.
