Let $A$ a non empty set. Which of the following conditions is sufficient because A is a countable set:
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$A \subset \mathbb N$
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$\mathbb N \subset A$
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exists a surjective function $f: \mathbb N \rightarrow A$ and $A$ is infinite
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exists a succession $a_n$ so that $\{a_n:n \in \mathbb N\}=A$
My attemp:
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is false because if the set $A$ is finite I can't find a bijective function fraom $\mathbb N$ to $A$
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is false because if I take $A=\mathbb R$ is not countable
but for the other cases?
Best Answer