问题4： 什么是无限集合？ 提示：有限集就是与某个自然数等势的集合 A set is infinite if it is not finite A set is infinite if and only if for every natural number the set has a subset whose cardinality is that natural number.提示：有限集就是与某个自然数等势的集合 A set is infinite if and only if for every natural number the set has a subset whose cardinality is that natural number. A set is infinite if it is not finite