本讲的前提是:
For the time being, all quantities occurring are assumed to be rational numbers.
假设我们所知道的数只有有理数,还不知道无理数的存在。
这里说的null-sequence 是rational null-sequence ,定义如下
继续
the second class is empty的例子请看
https://en.wikipedia.org/wiki/Completeness_of_the_real_numbers#Nested_intervals_theorem
的Nested intervals theorem部分