Through the analysis of the second power Cartesian product of natural number set N——N×N and the thirdpower Cartesian product of natural number set N——N×N×N,obtains the conclusion that they all have the bijective relation to natural number set N,it means that the set N×N and the set N×N×N are all countably infinite.
通过对自然数集合N的二次笛卡尔积运算———N×N和三次笛卡尔积运算———N×N×N的详细分析,得出了它们与自然数集合N之间都存在双射关系结论,即集合N×N和集合N×N×N都是可数无穷的。