Q: Let A,B≠0 and let f:A→B be an onto mapping. Then if A is countable then prove B is countable.
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Originally Posted by jas_viru Q: Let A,B≠0 and let f:A→B be an onto mapping. Then if A is countable then prove B is countable. This is one of the most important theorems in theory of cardinality of sets: . Now any subset of a countable set is countable. Because That means that .
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