下記で ”assuming the axiom of countable choice, a set is countable if its cardinality (the number of elements of the set) is not greater than that of the natural numbers.” google訳 ”可算選択公理を前提とすると、集合の濃度（集合の要素の数）が自然数の濃度より大きくない場合、その集合は可算です。有限でない可算集合は可算無限であると言われます。”
これ 百回音読してね ；ｐ）
(参考) https://en.wikipedia.org/wiki/Countable_set Countable set In mathematics, a set is countable if either it is finite or it can be made in one to one correspondence with the set of natural numbers.[a] Equivalently, a set is countable if there exists an injective function from it into the natural numbers; this means that each element in the set may be associated to a unique natural number, or that the elements of the set can be counted one at a time, although the counting may never finish due to an infinite number of elements.
In more technical terms, assuming the axiom of countable choice, a set is countable if its cardinality (the number of elements of the set) is not greater than that of the natural numbers. A countable set that is not finite is said to be countably infinite.
