Note that if any two sets are separated by a function, then they are also separated by closed neighbourhoods; the neighbourhoods can be given in terms of the preimage of f as U := f−1 and V := f−1, as long as e is a positive real number less than 1/2.
注意如果两个集合是由函数分离的，则它们也是由闭邻域分离的；邻域可以從 f 的前像給出， U := f-1 和 V := f-1，其中 e 是小于1/2 的正实数。